This could be due to the computer not being totally clean, you may still have something called a 'dropper' on the machine, this means someone can deploy malware/viruses/spyware and execute it on your machine.
Can you see any drives when you go to my computer, can you go to Start-Run and try Diskmgmt.msc then hit enter, it will show any drives you have.
Ideally, you'd take this to be repaired or if you have knowledge of computing hopefully you've created an 'image', have you tried to use system restore to a date prior to realising virus issues? It is possible that the restore points have been infected too though.
Do you have a sticker on your computer with a Windows XP serial number? If not... you may well be stuck as the next option is to telephone Microsoft, they will do a phone validation.
Best of luck, if you want remote support please contact me @ support@BerksIT.com - I will connect before to diagnose before quoting you a price.
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