As you have tried the suggestions so far without success it could be that you have picked up a virus as Alison556 suggests.
Sometimes it is just not worth the time and bother of trying to trace down these problems when a reinstall would be quicker. Providing you take the right options you should be able to restore your system including access to your programs.
This process goes something like this - Please do a search first to check this is correct
XP Repair install
1. Boot the computer using the XP CD.
2. When you see the "Welcome To Setup" screen, you will see these options
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 using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
3. Press Enter to start the Windows Setup.
To Repair Install, press ENTER
To setup Windows XP now and Repair Install , press ENTER.
Do not choose "To repair a Windows XP installation using the recovery console
4. Accept the License Agreement
5. Windows should find your XP installation.
Select it and press R to start the repair. If Repair is not one of the options, END setup.
6. The necessary files will be copied to the hard drive and the system will reboot. Do not press any key if you get a message to boot from CD. Setup will continue as though doing a full install but will still allow access to your correctly pre loaded applications.
You should then run a full anti virus check and spyware check