While I've never used McAfee, friends tell me it suffers the same problem I've always found with Norton: *massive* memory hogging. If you value your sanity, don't install any Norton version more recent than 2002 on a machine with less than 512MB of RAM.
Also with McAfee, bear in mind that a couple of months back, they unleashed a defective update to their virus definitions, which caused many people to inadvertently hose their systems. (Then again, you might see this as a good thing, as McAfee has been 'predisastered'. And there's no reason Norton couldn't do the same thing tomorrow...)
My personal gripe with Norton is that it doesn't play well in a security-conscious environment. I always try to have the day-to-day usage of a Windows PC done under a Limited User account (I consider this a simpler and more reliable security measure than any antivirus, BTW) - but Norton's automatic LiveUpdate doesn't support this setup, it will only work when you are logged-in as an Administrator-level user. To undermine security best-practice in this way is, IMHO, unforgiveable of a security-focused application.
At home, I use AVG Free Edition. While free AV applications seem to have a whiff of stigma about them, this one has really opened my eyes. It updates itself faithfully every logon, is simple to use and configure, and the memory usage is the lowest I've yet encountered. Even while scanning it's possible to continue working at the same time at a reasonable speed. And yes, its detection-rate is up there with the biggies!
Others I've tried include BitDefender Professional Edition, which worked pretty smoothly and has a super-high detection rate (so I read), with medium memory requirements; and Avast! free AV, which although it has the sexiest interface of the lot and good hit-rate, I found another very heavy memory-hog.
/rant over - hope it was some help