Here's the solution, but it entails reformatting (deleting everything from your hard disk) so back it all up before doing this.
Reboot - press whatever button you need to enter BIOS (Setup) - if you don't know, AS SOON AS the computer boots, press F1, F2, Insert, and Delete simultaneously.
Find the option in the BIOS that lists boot devices. Find the first boot device, and make sure that it is your primary CD-ROM/DVD-ROM/CD-RW/DVD-RW drive. If it isn't, change it to that.
Place your Windows XP CD in the primary CD drive.
Exit the BIOS, save changes, and reboot.
During the reboot, it will briefly display a message such as "Press any key to run Setup...". Press a key, and enter Windows XP setup. Follow the directions, reformat the drive in NTFS format, and reinstall Windows 2000.
That is the best, cheapest, easiest, and most definitive solution - a good reformat will often help Windows performance. Just keep in mind that you need to back up any files that are important and that you will have to reinstall everything that is presently on the computer.