Prefetch is a form of a way of memorising the things you do on a regular basis. So for example if you always launch your browser, open the email program and other programs at start up then Vista stores some of the information in prefetch so that it can get to it quickly and speed up the boot process.
It uses a tiny amount of disc space to do it (2-4MB) so there is no resourse issue. Some people advise clearing out the "store" but that advice is not agreed by everyone, prefetch will after all just fill it up again.
Do you also use Readyboost using an external flashdrive, if you do with 4Gigs of RAM it is not really needed.
30-35% of idle RAM usage is not unusual, in fact quite normal. It will depend a lot on what you have running on your PC. Vista will always take advantage of available RAM and as you will know needs a fair bit of it for it's own use.
As to the problem, I regret I cannot help any further.