stand-by mode problem

Hi! I would appreciate anyone's help in resolving the following issue: If I
set the time limit for my PC to enter stand-by mode to more than 15 minutes
or more, it does not go to stand-by. It works fine for 10 minutes (or less)
and for using the "Start" - "Shut-down" relevant option. I have disabled any
programs running in the background that I could see (even antivirus) and also
I tried disabling my network to no avail...
Any ideas?
Thanks in advance!