Pc Not Going into Sleep

[smu122]

Just recently i started to have this problem. when I attempt to put my computer in sleep mode, the monitor goes to sleep/black but the PC stays on. It just hangs, and the only way to get it back is to reboot the pc from the switch. I updated my video drivers but it didn't solve the problem. I haven't had this issue before.. An ideas for a solution?
Thanks for all the help..

Specs:
Windows Vista Home Premium 32 bit
Intel Core 2 Duo 3.0ghz
Nvidia 8800gt
2gb ram