I've been having a lot of problems lately with one of my PC's that is starting to get really annoying. The problem is happening when i shut down my PC and i go away for a minute or two its back on windows is loaded and everything is running. I don't know what this could be at all just wondering if any one has had this problem in the past and fixed it.

This PC is:

Enermax Liberty 620 watt model= ELT620AWT
4400X2 939
2 Gigs G.Skill
DFI RDX-200 CF-DR
ATI X1900XTX

Things i have done so far newest BIOS
Checked settings in BIOS nothing is really leading me to anything
New OS install a few times
MemTest+ 16 passes ran fine
Not overclocked because i thought that was the problem but thats not it.

Any help is very appreciated