Help Designing Your Robot

As said in sociatyofrobots.com, designing is the most important stage of robot building. I spent no time designing Turpis, and that's most likely why it turned out so bad. However, I'm an awful artist, and I need to find a more efficient way to plan. I've tried my hand at Sketchup, and it is uncooperative with my computers. Are there any easy-to-use free software out there?

Thanks,

Brennon