I am an audodesk inventor user - I appreciate that most on here would be stuck with a several thousand pound fee however I am informed there is a program called open inventor which does much of the same. The program is full 3d and is plainly fantastic.
Ive used many software packages including the ones that cost thousands, and for the joe public, and even more, id highly recomend sketchup.
There where many issues with SketchUp and Vista, i did get it working eventually.. then they issued a patch fixing it.. and it went crazy again.. thankfully its sorted and should work fine now!
some graphics cards still have troubles, but for help with that see the support on the sketchup page.
For free software you cant beat it, and if there is someone willing to go one step further, you can download Kerkythea which is a free rendering package (makes extra swanky models in sketchup look even better and also let you place true lighting effects) http://www.kerkythea.net/joomla/
I'd start off by getting a tape and measuring in real life the rooms essentials, i.e. lengths of walls, distance to doorways etc and putting that down on paper first.
Then for the sketchup part, there are a few free tutorials online that help get you started and talk you from step to step. It all uses a few simple buttons and visual cues. You can change the units if you dont like millimeters or inches
If you want to be really lazy and just copy and paste in some things (handy sometimes for everyday objects like beds or chairs for your cinema) Then there is a growing database at http://sketchup.google.com/3dwarehouse/