All I will say is: safe mode.
To get more detailed information, try Googling. I could help you more, but (as a few other members said in another thread) we technically don't really know whether you're telling the truth or trying to break into a stolen computer. Even if this is a legitimate thread, it's a public forum so anybody could read the response and use it for malicious purposes.
Although if safe mode doesn't work, I think your best bet would be to somehow reinstall Windows. Another option if you can't reset the password would be to install Linux on it (especially if it's just going to be a secondary computer. And if it's older, Linux should run very nicely on it).