Is the User Account you are using an Administrative Account?
You can only delete a User Account if you are an Administrator.
There is no real difference in User Accounts between XP and Vista. There are terminology differences, but the LImited User in XP and the Standard User in Vista cannot delete an Administrator Account. If you try in Vista, a pop up window will appear with listed Administrator accounts and you have to provide the password for one of them. You also cannot delete the account you are currently in.
Click Start->Control Panel. Then in the upper fight hand corner, under User Accounts and Family Safety, click on Add or Remove User Accounts. This takes you directly to the Add/Remove Window. You should be asked to click on the Continue button by the UAC, then the window opens. This window then lists all the current user accounts, and whether they are Standard or Administrative. Click on the User Account you want to delete and a new window opens which will let you delete the account. When you click on delete a new window opens up giving you the option of deleting the account and files (Delete Files), account only (Keep Files) or cancel.
If you click on the button to either delete files or keep files, you will asked to confirm your decision (Delete Account), and after confirming, you will then be taken back to the add/delete window.