Vista Tutorial - Areo turned off and won't come back on

Hi,
We have gone on vacation and changed the time zone on my computer. When we
got back and changed it back to our time, I forgot to shut the computer down
and it went into standby. The next day when I start it up again, areo tured
itself off and won't turn back on. It's worked fine untill now. I havn't
installed any programs lately. I first I thought that it was my AV program
that was causing the problem (since it was using the most memory), but I've
uninstalled it and reinstalled it. (the AV program is AVG free and it's not
installed right now)

My computer is a Dell Insprion 1501 with Vista Business.
I can live without Areo, but I'd like to have it.

Thanks,
Blair