well, id back up the files to an external hard drive or similar. Then run a sfc/scannow. Let me explain how this works.
Insert your windows CD. Now click the start button then click on "Run" (or the windows key + R) to open a RUN box.
Type in cmd and then hit enter.
Now you will see a black box with a blinking cursor. type in sfc/scannow and hit enter on your keyboard.
Wait for it to complete the process and then restart the computer.
Then see if you have the same issue happening. If so, you might want to read and follow the instructions in the following thread:http://www.tomshardware.com/forum/244555-45-performing-repair-reinstall-winxp-tips