Windows XP screensaver problem...

Lately, I've been having problems with my screensavers. Some work and some don't. When I test them out (preview) they come up just fine with no problems. But when I leave the room and come back later the screensaver doesn't come up. I check the control panel and it says that there are no screensavers applied. I'm not sure if it's a fluke with Windows XP SP2 or just a video problem...? The kind of screensaver I want to use are just your run of the mill pictures of Ireland or Australia. Nothing too extreme for my computer or monitor. What do you guys think??