Need spyware help

Hi all:

I've had my Mac about three years now, and just picked up my first
adware/spyware bug.

It pops up a fake (but real looking) Safari alert message urging me
to go to various websites to buy programs - Errorprotector.com  Etc.

I tried a system scan using 'VirusBarrier X' but my copy is expired &
I haven't updated it for maybe a year ...    I also found & tried
Macscan.com on the web. I tried using their Demo version but it isn't
finding anything.

What are the best current virus scans out there, and anything good thats
free?

Thanks, Phil