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.