Personally I prefer this over AVG - it talks to you when it updates!
NOD32 is consistently rated the best for detecting new and unknown threats (via behavioural analysis), and for the very little load it puts on the system, and fast scans. It also seems to me to be the best and fastest when doing updates. I buy mine via the UK site, but you can buy from the Spanish site if you prefer (the product can be had in most languages).

The latest version of NOD protects against Spyware and other Malware threats, as well as Viruses and Spam.

I personally use Eset Security Suite for Anti-Virus/Firewall combined (David S.)
Avast www.avast.com
Paid AV Products
NOD32 www.eset.co.uk
Useful links to dependable security software
Free AV Products
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Light and fairly speedy. Update - my equal top free choice.
Avira www.avira.com
Note - I no longer suggest AVG free version, due to the number and severity of problems I've seen it let through recently (updated 11/11/2009)
Beginning to get well regarded. This now my equal first choice with Avira.
Microsoft Security Essentials
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