Some of the commercial anti virus/ malware packages are worse than the viruses themselves in my opinion.
My friend swears that only commercial ones are reliable. Norton and another package he installed on his brother's computer have virtually rendered them so slow and unwieldy they were practically unusable. Of course he had to install Vista as a OS and it seems to spend all the time downloading updates, he rarely actually uses his computer for anything useful. Talk about watching paint dry! Personally I use free AVG, Malwarebytes and crapcleaner. All relatively fast, compatible with one another and simple (with XP) at least.