Registry "hive" corrupted message

My brother recently dropped his PC off at my house because of an error he keeps getting...
It's a decent PC that I built for him a couple of years ago, running XP Pro x64

Basically after logging the following error message pops up:

Title: Windows - Registry Hive Recovered
Message: Registry hive (file): SOFTWARE was corrupted and it has been recovered. Some data might have been lost.
Buttons: [OK]

Updated the virus signature file and performed a full scan (ESET NOD32) everything came back clean
checked for mal and spyware and everything came back clean
I've gone though and uninstalled all the needless applications

I tried installing a windows service called "User Profile Hive Cleanup Service" but it didn't fix the problem

I've also done a hard drive scan on all 3 drives and they came back clean, and I defraged all them for goo measure.

no matter what the error continues to pop up.

the only solutions I've seen posted about this error are to either re-install windows or create a new user profile and blow away the old one, though neither one of those options are all that appealing to me.

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has anyone here run into this issue and discovered a reasonable solution?