If the below fails you can try a Repair. I can't help you with that. Just the Website.
http://www.michaelstevenstech.com/XPrepairinstall.htm
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Try using System Restore in Safe Mode.
1-Boot to Safe Mode by tapping the F8 key while booting
2-Choose safe mode, Press Enter
3-Select operating system, Press Enter
4-Click Administrator Account
5-Windows is running in Safe Mode, Click YES
Click Start - all programs - accessories - system tools - system restore.
Restore to an earlier date.
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If that fails try Last known Good Configuration.
Turn your computer on and instantly start Tapping the F8 Key and don't stop
until you know you have a new screen.
Select last known good configuration.
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Sometimes I've been told a virus can hide in the System Restore, so you turn it off.
Only do this if all attempts at using System Restore fails.
Start - Control Panel - Open System Icon - System Restore Icon - Check mark -
Turn off System Restore on all drives - Restart your computer.
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Is your computer protected? If you did it harm and you need to Format your hard drive,
Get something to backup your computer after you use the Format and it's clean.
I have Acronis True Image 10 and a External Hard Drive
(Maxtor One Touch)
Acronis now has a newer version 11
Acronis 10 backs up to what's called It's Secure Zone.
I backup Acronis also to the external hard drive.
If the computer won't boot normal you insert the Acronis CD and press
Start. It goes to and open Acronis for you to run your backup.
Acronis now has 11
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Spyware Blaster
http://www.javacoolsoftware.com/spywareblaster.html
Try Ad-Aware
http://www.pcworld.com/downloads/
A free anti-virus
http://free.grisoft.com/doc/2/lng/us/tpl/v5
For windows xp you needed Service pack 3 from Microsoft in your Updates.
cg