For example some AV's offer a P2P(peer to peer)shield. Of course, if you don't use P2P, this will be a complete waste of resources.
Some AV's allow a custom install, allowing you to pick which element you need, this helps for such situations.
Always factor in your system before hand. Many AV's are large and always busy, and can be a strain on your RAM or CPU that are not up-to-date, since they are running in the background.
Do you use web-based mail? If so, your mail is already likely scanned, so a mail shield is not needed.
This is only a couple examples of what I meant by "needs", you know your PC best, so only you can say what you need.
Lets be frank about surfing habits, this doesn't require an answer;)
Do you surf adult sites, especially free ones? Just like in real life, frequenting the dirty side of the Web, you can catch something nasty. Same for P2P, or any downloading from any anonymous source.
Do you visit mostly safe sites you know, or surf where the wind takes you? I could go on all day with this, just Google "safe surfing habits" or something similar for more detail.
So you have to, basically balance what your computer and connection speed can do, VS. what your surfing habits are. Lastly, don't treat whatever AV you chose as the final answer. Help your AV help you by;
1:Full backup-system image+regular, preferably on an external drive.
2:An external Firewall, like what comes with a router, if you are on broadband (always connected from startup)
You can take security WAY beyond this with many free or paid options, but this is a good bare minimum.