Formerly sysinternals.com, Microsoft hired Mark Russinovich and now owns all of the Sysinternals tools. Thankfully, they are still free. I HIGHLY recommend these tools.
Mark is probably the smartest programmer to ever write software for Windows.
http://www.microsoft.com/technet/sysinternals/default.mspx
My favorites are ProcessExplorer, Autoruns, and TCPView.
