Firstly, you dont have to reinstall but just do a repair - in any case your data files will not be lost either way but you may have to reinstall some programs. Anyway, sounds like your registry is corrupt - have you tried "last known good" when you press F8 - or perhaps a "restore point" (thats if you can!).
To perform a repair install follow the steps below:-

Repair Install

NOTE: A Repair Install will replace the system files with the files on the XP CD that is used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.
NOTE: Factory supplied Restore Disks will not work to do a Repair Install.

NOTE: If possible, Before doing the repair install copy the wpa.dbl file to a floppy and after the repair copy/paste the file to the C:\windows\system32 folder, This will save doing a re-activation of the OS. But only if no Haredware changes are made.

1=Set BIOS to boot from the CD/ROM as first boot device
2=If the prompt to "Press any key to boot from CD/ROM appears (it should not on a repair Install)(Do so now as you have only seconds).(Ignore if prompt appears on any reboot)
3=Windows setup, Loads files.
4=You will see Windows XP home or Pro setup.
5=To setup windows press "Enter."
6=Press F8 to accept license.
7=To repair selected Windows Installation press "R."
8=Copies files to the Hard drive. 9=Computer will re-boot--Do not use the "press any key" command at the prompt.
10=follow the on-screen instructions..
11=Enter the "product Key" (Note- It is possible to enter a different product key from the one that was used to install the OS you are repairing but you will run into Licensing problems with MS.
12=The computer may re-boot again during the install, but do not use "the press any key" command at the prompt or you get to do it all over again.
13=Follow on screen instructions.