Strange sleep issue

If I tell the computer to sleep it will, but it will not go to sleep on its own. I can use system restore and it will but as soon as I open the power options menu it stops working again. Any suggestions would be greatly appreciated.

amd Phenom II x4 810 2.60 GHz cpu
8Gb kingston hyperx ram ddr2 1066 (240-pin)
gigabyte ga-ma790fx-ud5p motherboard
windows vista home premium 64-bit
xfx ati radeon 4890 1GB DDR5 2DVI PCI-e GPU

Welcome
Possibly this fine tutorial will help for both Vista and Seven
http://www.vistax64.com/tutorials/63567-power-options-sleep-mode-problems.html
Also
Try with a clean boot and see if that helps. If it does we can use the Clean boot procedure to solve
http://support.microsoft.com/kb/929135

Make sure that you have the most current BIOS version

Make sure graphic drivers are up to date.