SVCHOST is the "servicing app" for a whole raft of stuff in the operating system. I've looked a few times when memory's getting tight to see what exactly is using up stuff, and a couple of times I've disabled stuff that runs automatically. On the whole, there's not a lot you can zap without removing some functionality that you'll probably want.
Certainly, Norton was a major offender on mine - that's now long gone.
The other "pain in the neck" on my two Windows machines is Apple. That loads all kinds of stuff into memory, mostly to phone home and check for updates, or to sit and watch in case I plug in an Ipod. I now kill that (it takes about three goes via MSCONFIG, but it will eventually give up). When I want iTunes - about once in three months - I start it manually then plug in the device. Afterwards, I have to go through the kill routine again to zap iTunes, Quicktime, and some "helpers". I also have to put up with it inviting me to try various other Apple software that I don't want.
iTunes is also abominably slow, probably to teach me that I really need a Mac. No chance!