An odd set of problems - full re-install fail

Hello helpful computer people

I have a funny set of problems which has culminated in me wanting to fully reinstall operating software. I will explain why I want to do this in a minute, but the main problem is that I can't manage to do it! When I boot up my mac from the startup disk I get a messed up display. The information is obviously all there on the screen, but it's displayed in weird jagged lines. This means that I can't make out where to click to continue with the installation. I also though that the disk drive was being particularly clunky sounding, but could be imagining that.

You can see what I mean here: http://www.flickr.com/photos/33449131@N02/7160853520/in/photostream

The problems up to this point have been really odd, with applications one by one stopping responding and finder crashing. Also the computer will sometimes not go to sleep when closed and the fan will go a bit mad. I have had particular problems with browsers crashing and generally the computer grinding to a halt with nothing but the off button having any effect.

I have tried loads of stff - reseting P ram, repairing disks, running cleanups on Onyx, removing programmes etc.

I want to go back to square one because things are getting a bit messy, it won't let me reinstall Safari becuase it thinks it's already there, but It doesn't show up on search!

I tried reinstalling Snow Leopard but realised that didn't really do much and am now going down the route of putting the original install disk in. I bought there computer second hand btw, so there is a possibility that the install disks are wrong, although unlikely because the guy was trustworthy.

Thanks in advance for all your help

Tim