Need Help: Win7 64-bit, can only use 3,5 of 8GB of RAM

A couple of weeks ago my pc started running slower, and I think I have narrowed it down to my pc suddenly only wanting to use a max of 3,5GB RAM, even though I have 8GB installed.

I'm running Win7 64-bit, so it's not the typical 32-bit RAM max thing, and just dont know how to fix it.

I have tried to run memtest86 to test if there was anything wrong with the RAM, but it didn't find anything.

Right now I just want to check if you guys can help. If not, I'll just reinstall Windows and see if that fixes it.