Do a repair install.
1. Put your Windows XP Install CD into your CD-ROM drive.
2. Reboot your computer.
3. Let your system boot off of the CD.
4. Let the Setup go through the first part of the Installation procedure.
5. When you come to the screen in which it says "Welcome to Setup." press Enter to Setup Windows XP.
6. Press F8 to agree to the End User License.
7. Let the Setup search your system for previous versions of Windows.
8. When the Setup is finished searching your system, select your Windows XP Installation and press the R key on your keyboard to start the Repair Procedure. In 30-40 minutes, hopefully you'll be back at the desktop.
Install two spyware apps and one anti-virus.
AdAware, Spybot, MS AntiSpyware, Pest Patrol, Spy Sweeper, Counter Spy, and SpyBlaster is the list you choose from for spyware.
Zone Alarm, Avast, and AVG from Grisoft is your list for av.