WinXP Registry - Reset from DOS?

I did a fubar thing recently. I installed WinXP, then Vista, and all was well. But I wanted my WinXP drive letter in WinXP to be C: drive instead of G: drive that it assigned. So I went into the registry HKLM\SYSTEM\MountedDevices and changed the "\DosDevices\x:" around to the way I wanted.

Well now Windows won't boot. It gets past the scrolling Windows splash screen, then it goes to the light blue screen with the little WidowsXP logo displayed in the middle.

Is there any way to reset my registry quickly and easily? And is there a way to rearrange my system drive letters easily?

Thanks.

Thanks but no go. It game me an error that is due to a corrupt operating sytem. Chkdsk ran through ok with no errors.

Thanks for the xp repair link. I actually fell on that myself, but looked like it was just resinstalling so cancelled and started all over. Since both XP and Vista were fresh installs, not a big deal, just another few hours of my life wasted. (Thanks Microsoft!)

Strange, because it was going through the whole install sequence it seemed.