Either there is not enough drive space on the drive the WINNT folder is installed on to create the profile, or the Registry setting is too small, preventing the Registry to house the additional entries a new profile makes.
So, if you can't log in at all, I would run the Recovery Console from the 2000 Pro CD, and check how much drive space is available. If it is low, delete some files to make room. Reboot and see what happens.
IF it is still low, then read this Technet article (Q124594() aboutthe Registry Size Limit and how to change it:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q124594
(please remove any spaces)
Hope this helps.