Used Physical Memory,what percentage is being used ?
You may have failing ram or not enough installed.
Download Memtest click here
Use Nero or similar,if you do not have Nero,Imgburn will do the same job click here and burn the Memtest download as an ISO image.
You may need to change your bios boot order to boot from cd as opposed to harddrive.
Once this is done insert the Memtest iso disc and reboot the computer.
This will now diagnose your ram,if this fails it will tell you of errors.
Let Memtest run at least 10 tests.