I'm assuming you want to preserve data on your system. (otherwise reformating and reinstalling windows might be easier)
This sounds like a GINA problem. If you're able to access the registry of your system remotely over the network from another 2K/XP/2003 machine, you can check this key:
If it doesn't exist, that isn't the problem. If it exists and it says something other than "C:\Windows\System32\msgina.dll", you need to either delete it or make it say "C:\Windows\System32\msgina.dll". This is not necessarily the problem though.
Another possibility would be the keyboard filter. See this MS doc: (similar story)http://support.micro...kb;en-us;317246
You could perform a parallel install:http://support.micro...kb;en-us;266465
I hope this helps. Good luck!