The expression 'You get what you paid for' comes to mind.
Firstly it is alleged that some of the disks from the front of magazine can themselves include a virus, secondly some of the new viruses (virii ?) will get into your computer without you opening any attachments (my checker has on a number of occasions brought up an alert screen when an email is merely displayed and not opened) so to use an online checker won't stop you getting a virus and spreading it.
The checkers that you pay for can be updated as often as you want, you can arrange for them to update automaticallt without you haveing to worry about checking, or they can remind you that it has been so many days since you updated your protection.
In my opinion the anti-virus software is worth every penny spent on it.