Pop ups on my iMac? Help please.

[BrunoMac]

I've had my Mac for about a year. (27" iMac)
In the last week I've had a couple of pop-ups appear. Seem semi harmless at least they aren't porn, but it never used to happen.
When I'd get pop-ups on my PC I would run ad-aware and it usually helped. Is there a scan I can do to search for anything weird on my iMac.
THANK YOU!!

 

[Hellhammer]

Have you checked that your browser is blocking pop-ups (in Safari, click Safari in Menu Bar and select Block pop-ups)?