All my Windows Vista-on based desktops have always had problems sleeping/turning the monitor off. I can usually pull off a scheme for a while after a clean install but eventually it will regress to the point where the schemes just flat no longer work. On my most recent format, a Windows 7 64 PC, I am now at this point. I set a power plan to turn the monitor off after 10 mins and go to sleep after 20, but the computer does neither after the appropriate amount of time. This also happens on my girlfriend's current Windows Vista 64 machine as well.
I remember hearing about the potential for various programs to interfere with sleeping, but I was wondering how I would go about diagnosing which program, or what in general, is interrupting my PC's sleep/monitor off cycle?