It really depends on what you do with your system. Modern multi core systems with multiple gigabytes of RAM have plenty of overhead to allow virus/malware scanners to remain active and not show a noticeable impact. In I/O bound situations though, virus scanners still have a major impact. Hard drives just haven't kept up with the speed gains in the rest of the system, so now you have multi threaded apps all trying to use a limited resource. Having a virus scanner active on the entire system is a huge mistake in a work environment like mine, where people are compiling large codebases. I don't have exact metrics with my current project, but on a previous one, virus scanners tended to slow down the compile time by a magnitude of 2-3. I remember having to talk to IT about it after they rolled out some new policies that had scanning on for the folders where we compiled from.

As for my own personal Windows installs, I don't bother. I also don't browse the web with them, or have any of them booted long enough to do much outside play Win32 specific games from time to time. In the past I went back and forth. If I used Windows as my primary platform, I'd probably keep something like MSE enabled. There have been too many high profile normal websites hacked in recent years for me to be comfortable browsing anywhere unprotected. Even if the browser wasn't vulnerable, Flash has it's own share of issues. I've seen some very locked down systems still get hit with MMO password stealing malware somehow. It's one reason why Blizzard and other MMO makers are looking towards two factor authentication so much these days. It's far cheeper for them to support the occasional lost/damaged token then it is to deal with cleaning up a hacked account.

And as to Doug's initial question, task priority should help. Isn't there a way in Windows to start a process at a higher level automatically?