I've always shied away from disabling services, even the ones that Black Viper says you can disable. You just never know if some software or other that you install somewhere down the line is going to require them. I read an article by some tech guy who said that you wouldn't believe the amount of problems that are eventually traced back to someone disabling a service and forgetting about it. I think maybe it made sense 10 years ago when people had a small fraction of the memory/CPU power we have today, but these days you're not going to make that much of a difference by disabling a few services.
One "tweak" I would definitely make would be to create a new user account and use that for audio production. There are some Windows tweaks that are worth doing, such as disabling Aero and all the other bells and whistles, and with a separate user account you can do that without worrying about it messing up your everyday Windows experience. You can also disable all of the startup and background programs you don't need. For example, in my main user account I have LiveMesh running, which backs up certain folders online as and when they're changed. I don't want that running while I'm using audio software. I also have Outlook sync and Quickbooks services running in the background too....don't want those clogging up resources.
It may be worth researching system tweaks from a Pro Tools perspective, since they are a lot more documented. Also, Pro Tools is so picky and temperamental that tweaking and optimizing has been delved into a lot more. One thing I always do, from having used Pro Tools in the past, is to turn off my real-time virus protection and my network connection (so basically your audio user account will not be connected to the internet). That seemed to improve things for me Pro Tools wise, not sure what benefits it would have in the context of Sonar.