I personally find paying for basic antivirus software a bit over the top nowadays. Avira/AVG/Avast is good enough in my humble opinion and not that different from the non-free products. What helps is a good maintenance regime: keep all your software up to date (i.e. use Secunia PSI - free again), don't install crap, use web browser with adblock (i.e. firefox + ADP).
Unless you connect directly to public internet (wifi at starbucks, 3G), personal firewalls - sometimes offered as part of your paid antivirus suite, are also not that useful.
However, it is entirely your decision of how much protection technology you use (the more of this, the more complexity and maintenance problems you will have as well). I'm a security specialist and use linux for internet banking, windows 7 + free avast + firefox + adblock plus for regular daily webbrowsing etc. So far it was sufficient to stop pretty much everything.