Active window changing...

I'm stumped, so I've come here for some help please! I've searched this
board, and can't find anything similar.

I'm working on my MIL's new Dell PC. She's had it a few weeks, we've loaded
some software, mostly games , Norton AntiVirus, Earthlink]. Here's what's
happening: My active window becomes inactive. This typically happens while
connected to the internet. It's the strangest thing - nothing else appears to
be happening; no pop-ups, no error messages, no other application opens (that
I know of).

I might be writing an email, and I've got to move the mouse and click on the
email window to make it active again. I've checked Task Manager and there are
typically 40+ processes running. I don't know what they all are, but
recognize some (like mm_tray, iexplore, msmsgs, etc.). I ran SpyBot and found
a few annoyances, deleted all. Problem still occurs.

Norton is fully updated and running. Any suggestions? I'm not an expert, but
my MIL considers me her tech support.

Thanks for any help you can provide!