Slideshow Screensaver not Starting

This has been a problem for me since Vista. I was hoping that once I upgrade to Win7, this problem would go away, but it didn't.

I set my screensaver to the slideshow that comes with Windows. The folders I use is on my only SATA HDD. The power scheme is the default setting.

Most of the time, the screensaver slideshow would not come on. Instead, the desktop would be displayed until the power scheme turns the monitor off. I set the screensaver to come on after 15 minutes, and the LCD monitor to turn off after an hour of inactivity.

Again, I had this problem already with Vista, but it's the same issue with Win7. Does anyone have any idea why the screensaver doesn't start after 15 minutes?

Thanks in advance.

Some kind of media running? Music, videos - any apps that could play them running, even if they are idle? Maybe pictures open in a viewing app?

You said, "Most of the time" which means sometimes it works. Try to think about what is different when it does.

I don't have anything running in the background, except Avast!

Sometimes the screensaver does come on, but I cannot think of anything different when it does and when it doesn't.

I just assumed that it's a common problem for everyone, but apparently not.