Mouse and keyboard freeze, and monitor not turning on.

Just built a new system (spec below). About a week after it was built, the keyboard and mouse started to freeze. I would shut it down and my monitor would then stay in stand-by mode and not come on. This tends to happen after half an hour of the computer being turned on. I've cleared CMOS, re-installed all components and nothing seems to work. Not sure if this is my power supply, mobo or ram and i've no spares to test. Any Help would be great.

Windows 7 (64bit)
Arctic Power 700W PSU
Asus M4A79XTD EVO 790X Socket AM3
Kingston 4GB (2x2GB)DDR3 1600MHz
AMD Phenom II X3 720
Western Digital 640GB
Sony AD-7240S 24x DVD±RW
XFX HD 4770 512MB GDDR5 Dual DVI