Ok, so last night everything was fine. But today, when I try to use the CMD-Tab option to scroll through apps, it pops up but just continues to scroll through apps and will not stop. It cycles endlessly. Also, if I'm in Firefox, if I try any dropdown, like a URL I've been to before, it pops up quickly but closes again. So, I can't click any dropdown's now. Any thoughts?? I'm using Mac OS X v. 10.4.11. A second question: How many times can you re-install Mac OS X on one machine??