One thing about those wireless keyboard/mouse setups, is that sometimes the mouse moves by itself. I've seen it plenty of times on different computers... And a moving mouse breaks the screensaver, of course. You may have seen it yourself sometimes: Just typing away happily on your computer, when all of a sudden you notice your mouse starting to wander off...
Mine does it sometimes, in the up/left direction every time... (And that's actually a wired Logitech mouse).
Troy