I've worked in the AV industry for over 5 years in malware/spam research.
There are some ok products out there which are free....but generally if you want something good you need to pay for it... remember you need to be updating your AV hourly.
I would recommend
Kaspersky
Sophos
or Trend-Micro
what ever you do stay away from symantec/norton, their detection rates suck and their time to detection isn't any good either.
If you want something to just remove spyware/ad-ware/greyware...
then you could download
spybot search and destroy and/or
lava-softs adaware.
You should be able to download them for free... but just rememeber they don't protect you from malware (virus's,worms,trojans)
hope this helps.