ERUNT - The Emergency Recovery Utility NT
Registry Backup and Restore for Windows NT/2000/XP
v1.0, 11/24/2001, Freeware
Written by Lars Hederer
Look for the latest version here: http://home.t-online.de/home/lars.hederer/erunt
He also has another utility at the same location called NTREGOPT, an NT Registry Optimizer. I haven't used them myself but they were advertised on the main home page as "must have" XP utilities. I'm a sucker for that 'must have' phrase so i snagged up on them.
I think the only way to 'selectively' restore your registry is through system restore. Go to Start> All Programs> Accessories> System Tools> System Restore. You can see if XP has created any specific restore points you can roll back to. Hope that helps.