Forgot Username & Password for Windows 2000 Professional

Hi. I have a desktop that I did not use for some time and now that I want to use it, I have forgotten the username and password. I have a very expensive health related software in that system and when I called up the company they said that it has to be formatted and the whole thing has to be loaded again. It is going to cost me a lot of money that I cannot afford. Please please please, if someone can help me, I would be forever grateful.
Regards.