MSCONFIG is the first port of call. That's the easiest place to turn stuff off and on.
If you mess with "Services" you can be more refined about how you do it, but if you screw up in there, you can cause yourself serious heartache.
The other place to look is "Task Scheduler". I had a very persistent bit of software that I thought I'd turned off, but it kept creeping back. It had put an entry in Task Scheduler to check every so often to see if it was enabled, and to turn itself on again if it wasn't. Sneaky, that was!
(Win 7, click "Start", type "Task Scheduler" in the box, click on "Task Scheduler" at the top of the box. When it opens, click on "Task Scheduler Library".)