I use AVG anti-virus, and have noticed that the latest version slows the boot-up process considerably. But it's worth the delay.

If you're using Norton or McAfee, you might consider ditching them and try Avast, Avira or AVG. All offer very good free versions.

Althouh as I've already mentioned, AVG is somewhat slow to load, it's still light years ahead of Norton or McAfee imho. And none of them hog system resources.