Don't bother. No, really, the few MB of RAM and inconsiderable CPU usage isn't worth either the hassle of research, or that constant nagging feeling whenever something runs a bit slow or doesn't work right that you broke it, even when it's unrelated.
The only noticeable difference you'll see from disabling most services, is that the thing they used to do doesn't happen any more. Two notable exceptions are the indexing service, which on a slow hard drive can potentially negatively impact speeds immediately after adding a lot of files, and Aero, which can run a bit rubbish on very-borderline graphics cards. Other than that, it's not worth bothering with. If your PC is being slow, buying a little more RAM, or upgrading your processor, will be orders of magnitude better improvements.
If you really have to see this for yourself, googling the name of a service will generally bring up one of several sites detailing its purpose, where you can then make an educated decision on whether to disable it or not - but you won't see any noticeable difference, unless the PC you're doing it on is well below the required specs. to run windows 7 on at all.