FWIW: About the only bona fide way that I know of...to repair unknown, unspecified registry errors...is to do a repair or clean install of XP.
You may find this interesting, How to recover from a corrupted registry that prevents Windows XP from starting - http://support.microsoft.com/kb/307545/
If a user has any inkling that there are specific items in the registry which need to be repaired or removed, then a knowledgeable user would then use regedit to effect such.
There are probably any number of "registry errors" of some sort on any system at any given time. IMO, that's just the way things happen.
Most of these do not adversely impact the capability of the system to boot or to function normally.
But...sometimes they do adversely impact such...or they impact the proper functioning of a program which the user wishes to use...and problems become apparent.
Note that the info is geared for "advanced users", not "any user."
Having said all that...you can check Event Viewer and see if there is any info there about what transpired. If so, there should be a record of an error that occurred at approximately the same time you tried to boot.
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
My last bit of info has to do with registry cleaners, programs which are sold to/used by many users who haven't a clue as to what a registry cleaner is supposed to do...other than the fact that the AD...yes, that was an ad...the ad said that it would "stop system problems."What system problems
...how it would define those
...etc. are totally omitted from the ad...and the user often doesn't care because he/she has no clue as to what the registry is, it's importance, nor how to even see what the registry might be.
Some opinions of others on the value of registry cleaners:
XP Fixes Myth #1 Registry Cleaners - http://www.windowsbbs.com/windows-xp/61015...y-cleaners.html
Should I Use A Registry Cleaner - http://aumha.net/viewtopic.php?t=28099
Langa Letter Testing 10 Windows 'Registry Cleaning' Software Packs October 10, 2005 - http://www.informationweek.com/story/showA...cleID=171203805
FWIW: When I've had registry situations that prevented booting, I just did a clean install or a repair install (I always try a repair first). I've not had success with using someone else's instructions for recovering/repairing the registry and I don't like to spend all day trying to solve something when I know a reinstall of XP will take care of it.
Maybe...maybe not, at least as far as figuring out which file (I like the assumption that only one file is involved, it's not necessarily true). The registry contains a lot of different bits of data which could be damaged or missing, for whatever reason. Definitely depends on whether an error message was written to Event Viewer.
Someone else may have a more positive, more correct assessment than I have of what's possible.
If you don't have recovery disks or a Windows CD...and you intend to use XP as your operating system beyond tomorrow...I suggest that you obtain either one or the other ASAP. They are the only band-aids you can use on your system when it's damaged and it's not hardware or malware.