If you'd like to completely wipe out XP and C: drive, just do this:
- Boot up with your Windows 7 DVD
- When it comes to the first dialog, hit
F10 to open up a command prompt
- Go ahead and
format c: now
Congratulations, you've just removed every trace of XP. Unfortunately at this point, your computer is no longer bootable because you've just clobbered your system partition. The good news is that you're already in the Windows 7 setup screen now, so just tell it to repair your system and it will recreate everything it needs to boot.
If you'd like to do it manually from the command prompt instead, you can do it like this:
Theoretically, that should fix your boot records. However, when I tried this once, it said it couldn't find any Windows installations to configure. Don't fret -- just reboot with the Windows 7 DVD once again and try a system repair.