New MB and CPU

If I get a new mobo and CPU, would I have to reformat my hard drive? or can I just hook it up and I still have all the files I had on it before?

Yes, you can just hook everything up to your new mobo without re-formatting. If you have XP, you'll have to re-activate, but that's all.

Umm I would definatly recommend that you reinstall windows if you install a new motherboard. Far too many issues can crop up later on if you do not.

If you just switch mobos you will both get problems and the system will run much slower than with a clean windows installed
You don't have to format the drive, you can just reinstall windows.
You can select delete the current windows dir and install windows in that dir