Better than the yahoo toolbar is the Google toolbar.
For a quick free fix, all I did was install the search bar from google
(click here)
http://toolbar.google.de/ It also does several other nice
things besides search and block pop ups....it's a very usefull tool, however
if you want you can get Norton Utilities from Symantec which blocks pop ups
also
(just not for free, I think about $40.00 US) it too has several usefull
tools, including ad blocking.
On another note, Ad-Aware is a very good product and is available for free
and/or pay version....the only difference between the free vs. pay version
is you will (with the free ver) have to manually update the spyware/malware
paterns just as you would have to update your antivirus patterns for any
good antivirus software program (though it is usually done automatically
with a paid for subscription)
Hope this helps!
Steve