cmyk:
That error you are getting indicates an oversized or corrupt system hive. Sianath is right, the hive is definately larger than normal but not too large to boot (>~10.5 megs). You are likely looking at corruption. If you can load the damaged hive with the registry editor for XP it has some built in functionality that will attempt to correct damage.
In regards to booting to recovery console to run chkdsk:
Set your CD rom first in your boot order in bios. Press any key to boot from CD when it comes up. Bios is the only thing that really has any control over what loads at these early stages of boot. You should be able to boot to either a w2k or xp cd without any trouble. The problems on your system will not affect this.
In regards to the 'load hive' being greyed out:
To load a hive in your registry editor in XP, you must have HKLM highlighted before selecting File | load hive. After choosing which file to load (winnt\system32\config\system) you need to give it a temporary name such as "test". After loading it should tell you if it corrected any errors. Closing regedit will not unload the hive. To do so you have to highlight "test" and select File | UnLoad hive.
Don't forget to get a chkdsk run on your drive as well.
The low-down on hives:
%systemroot%\system32\config\system is your system hive.
system.alt is an identical copy that's updated simultaneously with system. It's simply a backup copy. If you get the missing or corrupt system error message, the OS has already tried to use this copy with no luck.
system.log is used to track if changes make it successfully to the hive. Basically anything that's about to get written to the hive is written here first. When changes are successful this will be empty.
%systemroot%\repair\system is a copy of the hive that is written just after GUI mode setup completes. It's old as dirt, typically useless for anything except booting long enough to fix the real system hive or do a restore.
%systemroot%\repair\regback\system is a backup copy of the hive made when you do a system state backup with ntbackup or choose to create an ERD with the 'also backup the registry' option. This hive, if it exists, is often your best bet to get running again with w2k.
C:\system volume information\_restore{ugly guid}\RP???\snapshot\_REGISTRY_MACHINE_SYSTEM is a backup copy of the hive made under XP whenever you create a restore point. This hive is often your best bet to get running again under XP.
Follow Sianath's instructions to get a chkdsk run, load your w2k hive in the XP registry editor.
The rest of you should probably just sit back and listen to the man. He's probably fixed this EXACT issue more times than he can count. You might learn something.
