Most pre-installed OEM Vista Computers have certain tasks set to repeat themselves, at a pre-set time interval. While most may be useful, there may be some which you don't want happening, but which may continue to annoy you.
 
It could be the annoying Easy Internet Sign-Up pop-up which pops up every half an hour, when you are connected to the internet, or the pop up for extended warranty programs or some similar such !
 
You may not find anything in the add remove programs. And even if you delete the program folder, you may still find it annoying you !

Log in as an Administrator > Start Menu > Control Panel > All programs > Accessories > System Tools > Task Scheduler.
 
On RHS, Under Actions > View > Select Show Hidden Task.
Search for the task under Active Tasks and double click on it. A new window will open.

On the RHS you will see the option to Disable the Task ! For example, if its the Easy Internet Sign-Up pop up which is irritating you, you will want to disable "InternetServiceOffers" ! You can disable any unwanted or annoying task this way.