If you want to change the times or frequency at which Vista creates system restore points, you can do so using the Task Scheduler (which is what Vista uses to manage these events). To edit this, you need to run the Task Scheduler. You can do this by going to Start > Search and then entering Task Scheduler and clicking on the Task Scheduler result.

Once you have the Task Scheduler open, you'll want to navigate to "Microsoft" -> "Windows" -> "SystemRestore" in the left hand pane in the window. Once you get there, you'll see an entry called SR Queued Multiple triggers defined in the pane in the upper right side. Double click on this to edit the system restore schedule and then click on the Triggers tab. You'll see two triggers (unless this has already been edited) that can cause the System Restore point to be edited. From this point it's pretty obvious, use the "New..." button to create new trigger points, the "Edit..." button to edit existing triggers for system restore, and "Delete" to delete triggers for the system restore.