Avast as suggested is good, i've used it earlier. AVG is ok, Clam AV is free, but it comes with a few false positives. Spent too many hours chasing those down
(uninstalled).
Commercial AV i'd go with would be F-Secure. Kaspersky is ok. Also some sort of whitelisting protection can be useful (it's built into windows, google it). If you can find something that sandboxes malicious code, go with that AV (i know panda had that before most others, but i dont know the state of that product today).
If you want to run software without worrying, get Virtualbox (or vmware if you have the cash), install the OS of your choise and play around, if you get infected, you can just revert back or throw away the image and begin again, either by copying the virtual harddrive image or reinstalling the OS on a new one.
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"It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring" (Carl Sagan)