I have installed windows 2000 professional edition on the top of windows 95 and I have been using it for a couple of weeks. Now I can not recall my password, what can I do to bypass the login screen. Is there a default login (username/password) for admin or domain admin that I can use? Do I have to do a reinstall?

If you have the machine logging into a Domain, then you can indeed use a Domain User or Administrator login. There are many posts with this same problem. I'd recommend you check them out. There are many different ways to obtain local account IDs and passwords.

I can not login to domain or administrator. Tried to reset the password for the user account at boot time, logout and restarted the computer, and I was prompted to enter password, I typed in the the password and was prompted to enter password again , the third time, i got an error message saying password invalid and system halted. Why the password I supplied was invalid. Now I can not even do a reinstall.

I would suggest asking your Domain Administrator to reset your password. Then you can login to the machine using your Domain account. Regardless of what you decide to do, you can always reinstall if you're really that desperate, but you'll need a Domain Administrator to add the machine to the Domain.