The error you are getting is due to the system registry being corrupted and the first step I recommend you to do is perform a scan of the hard disk for errors from the recovery console, the second step to take is to perform a manual system restore of the registry as explained in this Microsoft web site.
http://support.microsoft.com/kb/307545
This is not for the beginner but if you try it you will learn a lot, but it would probably be easier to perform a repair install.
If you don't have a CD you could repair it by putting your hard drive into another working computer and use the command prompt from there.