Bug or feature ? Explorer windows disappear.

[Frank]

I haven't been using XP for too long, but I noticed
something weird: after a while, 'Windows Explorer' windows
disappear from the taskbar. They still show in the alt-
tab choice, and when I select one it just reappears on the
taskbar as if I had just opened it (but remains in the
state I had left it, i.e. opened folder trees, etc.). I
find this annoying because I usually keep one or more
explorer windows handy, and now I have to actively look
for them, so it kind of kills the whole idea of having it
on the taskbar for fast switching ! :P

I work on other computers with XP, but I haven't worked
long enough to notice if they do the same thing.

I didn't see it actually happening while I was working on
it; it happens overnight, or when I watch a dvd in full-
screen mode, etc.

It shouldn't be because there are too many apps opened and
there isn't any space left on the taskbar, because I tried
enlarging the taskbar and they still did not appear.

FYI, it's windows XP Pro SP1 with all the updates
available as of last week, running on a dual Xeon 2.4 HT
and 1GB ram.
"Special" settings: XP classic look and classic start
menu, app grouping disabled on the taskbar.

So, bug or feature ?

thanks for reading my ramblings.