It's nice when a Windows program asks you whether or not it should run at startup, but there are a few out there (I'm looking at you, Apple software updater) that feel the need to hide themselves in Task Scheduler, away from the more common areas of the user's Startup folder or the registry. Fortunately, disabling or deleting these tasks is a simple process.
To run Task Scheduler, you can type its name into the Start Menu's search box, or track it down in Programs -> Accessories -> System Tools. To see what tasks are currently active (that is, enabled, not those currently running), select "Task Scheduler (Local)" in the window's left-hand tree view. Then, in the central pane, scroll down until the "Active tasks" list is visible. You'll have to double-click on the tasks you want to change, as the right-click context menu is not available here. Then it's easy to disable the task or remove it completely.
CCleaner, my preferred system maintenance tool, does a good job of picking up tasks from programs that you've installed (as well as in the registry and Startup folders), but it doesn't show you tasks set up by the operating system. This is a good idea, because the last thing you want to do is disable something critical, but what Microsoft believes is important and what a power user thinks are not always the same.
Once such example is WinSAT, or the Windows System Assessment Tool, which for some reason, feels the need to run each week at 1am. If you're like me and often work late, the last thing you want is WinSAT to run, temporarily disabling Aero (for Windows Vista / 7) and freaking you out ("Did my video card just die?"). It doesn't do anything important -- at least, nothing that justifies running it once a week -- so feel free to disable it.