To delete a user profile.
You must be logged on as a computer administrator in order to complete this procedure.
Open System in Control Panel.
On the Advanced tab, under User Profiles, click Settings.
Under Profiles stored on this computer, click the user profile you want to delete, and then click Delete.
To open System, click Start, point to Settings, click Control Panel, and then double-click System.
Have you tried doing it from Safe Mode?
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Select both boxes.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
If they don't work you could try repairing windows by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)
As for Virus scanners it is a matter of personal choice. For a paid version I like NOD32 from Eset, as a free version I like AVG Free from Grisoft.