Do I have to reinstall xp or just replace files?

I'm planning to switch AMD sempron 2500 to p4 2.8 ghz+motherboard(keeping the rest) and keeping the same hard drive. Question is can I just do the repair option in xp pro setup to replace just the system files without formatting or do I have to format C drive(I have 1 hard drive with 2 partitions) keeping the D drive? Also, should I uninstall the programs before setup such as the games stored in D drive or just delete them after installation?

You wont need to do any of that.

All you should have to do is uninstall all your old motherboard drivers, shut down, replace components and reboot, reinstall new drivers..

this should be all you need to do. A Repair if any problems arise should be your only need.

Just unistall all of the drivers before switching. After switch install new driver and activate windows.

You dont need to reinstall, but imo a fresh install with new mobo/cpu is always a good idea

You could consider yourself VERY lucky if you didn't run into any major problems, without at least performing a repair install. Like Red_LightRanger suggested, a clean install is the preferred method, especially when switching from an AMD to Intel setup (or vice versa).