Create a new User Profile and see if it works in that.
If it does your User Profile has been damaged and you need to move all your Data & Settings to a new user profile you repair your existing profile.
If it doesn't you have a corrupt Windows Install and need to repair it. So you can follow the directions here
http://support.microsoft.com/kb/315341
[i]Method 1: Start the reinstallation from Windows XP
To reinstall Windows XP by using Windows XP CD, follow these steps:
Start your computer.
Insert the Windows XP CD in your computer's CD drive or DVD drive.
On the Welcome to Windows XP page, click Install Windows XP.
On the Welcome to Windows Setup page, click Upgrade (Recommended) in the Installation Type box (if it is not already selected), and then click Next.
On the License Agreement page, click I accept this agreement, and then click Next.
On the Your Product Key page, type the 25-character product key in the Product key boxes, and then click Next.
On the Get Updated Setup Files page, select the option that you want, and then click Next.
Follow the instructions that appear on the screen to reinstall Windows XP.
If you successfully completed the reinstallation, congratulations. You are almost done. Please continue to the "After you reinstall Windows XP" section to finish.
If you received an error or if the reinstallation did not finish, try method 2.
Method 2: Repair install of Windows XP by starting your computer from the Windows XP CD
Note If Windows XP was preinstalled on your computer, you may need the installation CD to reinstall. Contact your computer manufacturer to make sure that you have the installation CD for a repair installation.
To reinstall Windows XP by starting your computer from the Windows XP CD, follow these steps:
Insert the Windows XP CD into your computer's CD drive or DVD drive, and then restart your computer.
When you receive the "Press any key to boot from CD" message on the screen, press a key to start your computer from the Windows XP CD.
The following message on the Welcome to Setup screen will appear:
This portion of the Setup program prepares Microsoft Windows XP to run on your computer: To setup Windows XP now, press ENTER. To repair a Windows XP installation by using Recovery Console, press R. To quit Setup without installing Windows XP, press F3.
Press ENTER to set up Windows XP.
On the Windows XP Licensing Agreement screen, press F8 to agree to the license agreement.
Make sure that your current installation of Windows XP is selected in the box, and then press R to repair Windows XP.
Follow the instructions that appear on the screen to reinstall Windows XP. After you repair Windows XP, you may have to reactivate your copy of Windows XP. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
310064 (http://support.microsoft.com/kb/310064/ ) How to troubleshoot Windows XP Setup problems when you upgrade from Windows 98 or Windows Millennium Edition
If you successfully completed the reinstallation, congratulations. You are almost done. Please continue to the "After you reinstall Windows XP" section to finish.
If you received an error or if the reinstallation did not finish, unfortunately, this article did not resolve your problem. For your next steps, you may want to ask someone whom you know for help. Or, you may want to contact Microsoft Support to help you resolve this problem. [/i]
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