+1s for RealBeast and sairasie's posts.
Here is a summary of my experiences with anti virus software (note Norton was a while ago):
Effect on system speed:
Norton - significantly slowed
McAfee - horribly slowed
Comodo - slightly slowed
MSE & Windows Defender - no noticeable affect
Ease of use:
Norton - no complaints, good selection of tools?? (can't remember that well)
McAfee - menus were a bit frustrating, good selection of tools
Comodo - easy to use, ok selection of tools
MSE & Windows Defender - very easy to use, few tools included
Protection:
Norton - detected some stuff, slow scanning (?? hours)
McAfee - detected some stuff, lots of false positives, missed a virus that Comodo then picked up, very slow scanning (7 hours +)
Comodo - detected some stuff, can't remember how fast it scanned (?? hours)
MSE & Windows Defender - some false positives, didn't detect anything (probably wasn't anything to pick up), fast scanning (~3 to 4 hours)
Reputation:
Norton - have heard lots of bad things about it, seems to be very unpopular, but highly regarded by CNET (note: would not recommend taking buying advice solely from CNET or PC Mag)
McAfee - mixed reviews, I have heard it is thorough, but also lots of complaints on speed
Comodo - seems to be smaller, haven't heard much about it
MSE & Windows Defender - have heard very bad things about its ability to detect viruses
Other AV brands of note I haven't yet tried:
Kaspersky
AVG
Bit Defender
Anyway, hope that was helpful - I based the above on personal experiences and reading reviews, so take it with a grain of salt and some research.
lux
EDIT: spelling mistake, paraphrasing