It sounds like your graphics driver isn't loading correctly. Boot into safe mode:http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true
Next, uninstall your graphics driver:http://www.playtool.com/pages/uninstallgraphics/uninstall.html
Reboot and then redownload the driver from your computer (or graphics card, if you built the computer yourself) manufacturer's website and reinstall it. You should hopefully be good to go.