Screen saver not starting

Does anyone have a handy checklist of things to look for when the Windows Screensaver doesn't start?

Last weekend I went to install security updates etc on a pretty standard P4/Gigabyte/XP Home system I put together a couple of months ago for friends (they only have dial-up). The screensaver will preview but not kick in.

Settings appear normal to me, I checked registry settings suggested on Kellys Korner, the mouse cursor isn't fluttering, although it is a MS wireless keyb/mouse combo, I'll try standard units this weekend to eliminate that.

Something must still be running in the background that is prevent the SS from starting.

The problem is the MS wireless keyboard/mouse, as per here:

http://www.mistered.us/wwwboard/messages/13271.shtml

I wasn't there long enough tonight to chase background processes, but a standard kb/mouse lets screensaver work normally.

According to Google searches, various people are having trouble getting MS wireless kb/mouse combos to work properly with XP...