Alt-Tab, the screensaver is disapearing somewhere
Yes this happens to me also under Windows Me. The aquarium seems to use Alt-Tab as an internal shortcut: "minimize to nowhere". The reason that you can't use Alt-Tab after that is because the aquarium is still running and the task key Alt-Tab should always be disabled by a screensaver when it runs. I suspect this to be a Win95/98/Me bug, since the whole task keys thing works differently under Win2K/XP.
memilm,
You can download my tool "Task Keys Reviver", so you don't have to reboot when this happens again.
http://home.wanadoo.nl/sfinx98/progs/keys.zip