I leave a video file on pause, and my player is set to disable the screensaver so as not to disrupt my viewing enjoyment.
There's probably some software program running that's disrupting this. The fun part will be figuring out which one it is. I'd boot into safe mode, and then start the programs you normally have running one by one. After starting each one, try to put the system into standby until you find the one that kicks it back out.
If it's not any programs you run, then it would probably be time to check for viruses and spyware/malware. Trojans and keyloggers could easily account for this behavior, but it's also best not to jump to hasty conclusions.
I don't know whether it is a hardware problem or software problem. My PC mysteriously resumes from standby mode like 3 or 4 seconds after it enters it. I unplugged all the devices except a keyboard and I even switched the keyboard with another one just in case it's broken, but the computer still resumes itself. It's just weirdest thing ever happened to my PC.
My system is a windows XP, geforce 6800gs, abit al8-v motherboard and a OCZ 520w powersupply.
Does anyone have any idea why this happens? Thanks!