You said you have set the Virtual RAM at two sizes, 800MB, and 1630MB. If it is 800MB change it to min 800MB, max 928MB and see what happens, If it is set at 1630MB, then you should have more than enough Virtual Memory.

Check that your RAM has not gone bad by right clicking on My Computer and select Properties, then select the General tab, it will tell you your processor speed and the amount of RAM that you have. If it is less than the 760MB that you expected, that could be the reason for the slow down.

If you open up Windows Task Manager (right click the task bar and select Task Manager) and click on the Performance Tab, how much memory is being used (the PF Usage)? If the PF usage is larger than 400MB, select the Processes Tab and click the "Mem Usage" to sort the programs by lowest to highest memory usage. Make sure that the "show processes from All users" is selected. What program is using the most memory and how much is it? Are there multiple programs over 60,000K? (Currently Firefox is using 48,780K on my system as a reference)

Your slow down could be caused by the fact that your hard drive is very full, try deleting anything that you can do without or buy a larger hard drive (a 80GB hard drive is $45 @ Tiger Direct, a 160GB is around $10 more)

One more thing, I have had this happen to me before, my PC was running very slow, but it was just that my antivirus was running a scan in the background. Make sure that nothing like this is the cause of the problem.

Hopefully one of these will point you into the right direction.