When I bought a new computer last year it came with Symantec's Norton Antivirus, which I've continued to use since then. However, as it's now coming up for renewal, and as reviews suggest that I could do better, I decided to try the ZoneAlarm Security Suite, which CNET is quite enthusiastic about.
So far ZoneAlarm seems to be working fine, and I suppose I'll pay for it when the 15-day free trial runs out.
As with Norton Antivirus, it offers the (recommended) option of scanning files whenever they're run or opened, but this seems excessive to me and I always turn it off. I scan incoming e-mail and every now and then I scan the whole hard disk. That should be enough, and so far it has been enough.
ZoneAlarm's junk mail filter works only with Outlook or Outlook Express, but I use Eudora and it has its own junk mail filter, so that's OK.