As both Ethel and Vehelfulguy suggest, there is point in paying for any security software. AVG updates and scans daily and consistently scores 100% in independent tests of virus detection.
The vast majority of the 'regulars' here in AB's Technology section, myself included, use AVG in combination with the free version of Zone Alarm. They deal with all of the real security threats to your PC but it's also a good idea to use something like AdAware Personal Edition.
Chris