Good Screen Recorder?

I currently use Screencast-O-Matic, but with unsatisfactory results. I’m trying out the free version of Fraps, but it’s a paid program which may have annoying limitations in the free version. Anyone know of a good, free Screen Recorder that doesn’t lag your computer?

OS: Windows 7
Processor: Intel Celeron (I know, the budget one) 900 @ 2.20GHz