There really aren't that many viruses for phones these days, so most antivirus programs endear themselves by offering other features, such as data backup and location services (see Lookout for a prime example.) They don't usually impede performance though, except temporarily as they scan installed apps.
For extra security, read the comments around these Mobilism posts. I lost all my data because whoever released Tasker here didn't crack it properly. And so far the mods haven't done anything about it.