I get this too. I'm on WinXP, but it used to happen on Win95 too. I think (although I've no evidence) that it's related to Netscape on my machine - what browser do you guys use?
Technically, I think what happens is that Windows' internal cache of "bitmaps" (ie icons, buttons etc) gets corrupted in memory - ie something overwrites the bitmaps' memory with non-graphical data - the stored versions of the bitmaps on disk aren't involved. Such memory collisions would usually be catastrophic, but as Dan Dare said, it doesn't seem to cause any problems at all, besides the untidy screen!
cbl.