As soon as you power on your computer, immediately
start clicking the F8 key and keep clicking until the Advanced Boot Options menu appears. Once you get into the Advanced Boot Options menu you should be able to use your up/down arrows to select (highlight) Safe Mode. Click Enter and Safe Mode should start. (You'll see the words Safe Mode on all four corners of the screen.) From Safe Mode I'd try to access System Restore and roll back your computer to a restore point before this problem first began.
How to restore Windows XP to a previous state
If a restore point doesn't fix thing, you might want to go through this article for other troubleshooting steps.
10 things you can do when Windows XP won't boot | TechRepublic