Kaspersky has the smallest footprint with the most up to date virus and malware solutions on the market. Not sure if there is a free version but if you were going to buy on, Kaspersky is the best, period.
Malware bytes is still one of the best for free. I just check and I can run the updates from them fine on my laptop. If you can't get the updates to run then chances are you are already infected and the virus is preventing you from running the updates. The best thing to do in a situation like that is to remove your hard drive and slave it to a known clean system and run malware bytes from the other pc. This is called an external scan and will get rid of things like boot sector virus problems and viruses that prevent you from running updates.
If you are looking for a free solution then I highly recommend AVG and Malware bytes together. This will get of most problems, you just need to update them.
Sophos is also great.
