but yes, I have seen/responded to numerous posts where a Trojan (usually) has hijacked a computer by changing the password(s) which you won't know, to prevent your use of it. Often this is to add your machine to a "botnet" for spamming or other purposes.

Often, the smarter ones don't do this as it, in effect, notifies you that they have control. Smarter ones want to have control/use in background, where you won't see it, so they can use for as long as possible before you take action to regain exclusive control.

Besides scanning & fixing with trojan/malware removers, there is also a small program which will let you over-ride the existing password & let you set new one of your choice.: http://www.petri.co.il/forgot_administrator_password.htm
Thread here:
http://forums.cnet.com/5208-6142_102-0.html?forumID=5&threadID=261601&messageID=2570271#2570271

Hope this helps get you working again. Be sure to run SuperAntiSpyware
&/or Malwarebytes Malware remover programs & perhaps double check with an online scan at Housecalls or other. Links for these in this forum sticky "Tools" at top of front page or also mentioned in various posts with links & instructions on how to use.
Enter into Advanced search :this forum & program name and "last 90 days" to find individual posts. Good Luck!!