JReece - you must have either a dv9000 or 9500. I have been looking at them, primarily because of the dual drives. Not too common on laptops. You must have had the second HD added, for HP usually puts two of a kind in their laptops with dual drives. Which ever, it gives you a BIG advantage. Since you complain how slow the machine is, I would lay bets that you are still using the Nortons Internet Security program. It is the source of may of the problems people complain about. My machines have all been stripped of Nortons and are running free alternatives.
Now, if this were my machine and I was going to take Vista off, this is what I would do to avoid burning bridges. Your drive with the factory Vista installation has a partition on it that is the recovery partition. Although you may not think you will ever want Vista, it should be preserved.
In your documentation somewhere it should have mentioned burning recovery discs, and it should have directed you to do that immediately. If you have not done that, then that should be your first step. I do not know how many DVD's it will take for Vista, but when my Media Center came with XP Media Center it took 4. These DVD's should be marked as recovery DVD's, put into jewel cases and put in a safe place. MPH and I wish we had a dollar for every post of someone trying to recover a laptop without them.
Next if you want to dual boot, keep in mind that drive 0 already has two partitions. My main drive no longer has the recovery partition because it now Vista, not XP MC, and I have only used 13GB. I have Vista, Quicken, Office 2007 Home & Student, Adobe Reader 8 and various utilities. You may be happier for several reasons either putting XP on the second drive and when you have it up and running, remove the Vista drive and store it. I still have the original XP MC drive with the recovery partition to my machine on a shelf.
Before you attempt to install XP, you need to go into device manager and make a list of every installed item in the list. Especially the video card, the modem, the ethernet port, the wireless adapter, etc.
Then you need to go onto the HP website, click on the lower left side button for Drivers and Downloads. It will ask for you model number, and give the product number that is on the white sticker on the bottom of the laptop. Not the serial number. When you products listing comes up, if you are lucky, it will ask which OS, XP or Vista and which version. You need to note the versions available. If XP Home is there, you are fortunate. Then you should be able to install XP Home relatively easily.
Download all the drivers you have identified and save in a separate folder. I have a download folder in my system that has all the downloads for programs and drivers in it. That way if I ever have to reinstall, I have it available and don't have to go hunting for it. You may want to rename some of the drivers for what they are, for HP uses a non-descriptive naming convention on its drivers. Once you have the drivers downloaded you are almost ready.
From this point on, we will assume that you are removing Vista from the machine in a non-permanent manner.
If you have data on the second drive, you need to move it to the primary drive so the second drive is cleared. Then turn off the computer, remove the primary drive and move the second drive into the primary position. These are SATA drives and are keyed to C: and D: by which bay they are in. In a desktop we just swap cables. When that is done, and you are ready with XP you will need to turn the PC on and insert the CD into the optical drive. You will get an error when the PC finds no operating system, but ignore it for now.
Unless you have an old Windows 98, 95, ME or 2000 disc around, you will probably have to buy the full install version of XP which will run around $200. The upgrade version will call for a previous Windows installation when it does not detect it on the PC, but, I don't think it will consider Vista to XP a recognized upgrade path. BTW, XP Pro to Vista Home Premium is not a recognized upgrade path either.
Restart the machine and you will get very early on down in the lower left corner some boot options. I don't have a dv9000 series to play with yet, but my zv5330 should be very similar as HP tends to be consistent. One option is to hit <Esc> to change Boot Order a second option is to hit <F10> to enter setup and the last option should be to hit <F12> to boot from LAN. The first option is the best bet, for you need to direct the machine to boot from the CD. Then you simply follow the CD's instructions to format the drive and install XP. XP will by default install generic driver for everything that it has to have. When it is finished the machine is very basic, and will probably have very poor graphics, but you need at this point to to create a download folder and copy the downloaded drivers from the Vista drive which is now in the secondary position into this folder. Then turn off the machine and remove the Vista drive.
Restart the machine, start Windows Explorer and go to the downloads folder and if the driver download is the normal HP softpak you will need to double click on it to exapand it. This is determined if it is an .EXE file. Then go into device manager, and one by one, double click on the devices and in the dialog box, click on update driver. In the next dialog box it will ask if you want to connect to the internet to seach for better drivers, for all updates at this point, check the not at this time box, and click on next. In the next dialog box check on Install from specific location (Advanced) and click on next. In the next dialog box uncheck update from removable media (CD) and check Include this location and click on browse. Now point the computer to the exact folder with the driver for this device and click next. XP will do the rest.
Then reinstall the programs you intend to use you should be ready. I apologize for the long detailed post, but it is not a difficult process, just involved.
I would at this point purchase another 2.5" SATA laptop drive and install it in the second bay as a data drive. At some point in the future, you ever want to revert to Vista, you simply swap out the XP drive for the Vista drive. It's not a dual boot but the bridge is still intact. If you want to burn the bride, then you format the Vista drive and turn it into a data drive, and if you want to revert to Vista, you will need the Recovery DVD's you burnd at the start of this process.