horbar,
FWIW, I run WinMe with OPP 3.0.543.5722 (431), and Avast with almost no discernable difference from the previous version of OPP. I do know that specific hardware/software combinations make a world of difference, however.
I've followed with great interest DavidR 's threads regarding OPP's heavy-handed file access during boot-up. Again, *I* didn't notice any significant slow down, but it was clear from those threads that he, and others, did suffer from the access loads.
I've been running OPP's anti-spyware, mostly just to see how it "feels", but I'll probably turn it off since I have a pretty tight ship and I regularly run 3rd. party scanners for spyware and the like. Lean is great, when it's the best (like OPP & Avast)
If you're willing to twiddle with settings and engage the great minds that frequent this forum, I'm sure that you'll be able to significantly improve your boot times without sacrificing too much (if any) in the way of prodection.
Best of luck.