Weird RAM proble, could you guys please help me?

Hello everyone, i have the following problem:

My PC says it has 4GB of RAM installed but only 3.5GB are usable, i have windows 7 home premium X64.

CPU: Athlon II x3 Unlocked to Phenom II X4 @3Ghz.
Motherboard: Asrock N68-VS3-UCC
RAM: 4GB DDR3 generic brand.
HDD: 1x 320GB SATA, 1x 250GB SATA and 1x 250GB IDE
GPU: XFX ONE 5400 series
PSU EM 500w PSU.

Windows recognized the full 4GB and the whole 4GB were usable but when i checked about 2 days ago it said that only 3.5GB were usable.... i was very confused so i stated to do a little research and i found out that my GPU might be using Hyper Memory so i tuned off my system, removed the GPU and when i wen't to see how much RAM was usable it said 4GB installed 3.25GB usable....so its not my GPU, if you guys need any more data for helping i will provided ASAP.

Thanks!!