Pop in the WinXP disc in the CD-ROM, boot up from the CD-ROM and select the repair option.

If you can still get into WinXP, use the System Restore feature (Start -> All Programs -> Accessoires -> System Tools -> System Restore).

Lastly, you can still hit F8 with WinXP. You could use the "Use last known good conifiguration option" to try solve any registry issues you may have.