Windows 7 64bit shows 3.73GB usable memory with 8GB installed

I was able to get it fixed by switching the RAM around a couple of times (first time didn't work, second time did), so maybe they just weren't seated properly the first 2 times.

Thanks everyone for the responses.

I had this problem....I have 7 gig and it was only seeing 5! before I noticed this I had messed about with MSCONIG into the boot tab, advanced options and unticked the box maximum memory and that worked for me.....