That sounds like a Userinit reg key that has been fixed incorrectly, altho the symptoms you describe are just a little different--maybe it is the Winlogon key.
In any event, when you can't get logged on to windows to fix the problem, the quick answer to your question is no, it isn't easily fixed if you've never done it before. There are three ways I know of to do it:
1. Boot from a CD using BartPE and then repair the registry.
2. Boot from a standard XP CD (not OEM recovery disks) to use the Recovery Console to repair the registry.
3. Take the hard drive out of your PC, hook it up to another computer running XP or higher and repair the registry.
Rather than give detailed instructions, let us know what you want to do, as it sounds like you will just take it to a shop if it is very hard to fix. Before you do anything, I would suggest you post about this in the MalewareBytes Anti-Malware support forum. This is the first I've heard of such a problem when using MBAM and they need to know about this problem so they can figure out what went wrong and correct it so it doesn't happen again to anyone else.http://www.malwarebytes.org/forums/index.php?showforum=41
They might also be able to give you detailed instructions on how to fix this after determining exactly what the problem is. That would determine what repairs need to be made to the registry. I'm sorry this has happened to you but I hope you won't blame MBAM. It's the malware that is using the userinit key and other obscure ways to start that is responsible--there is always a risk that fixing such problems will go south on you.
Edited by Papakid, 31 March 2009 - 10:13 AM.