PLEASE HELP! Some programs in Windows 7 will not start?

This is extremely frustrating. Previously I had thought that only Firefox was not working, but know however, many programs aren't working either. These include iTunes, Quicktime Player, Firefox, Youtube Downloader etc. When I click on them, they load for a few seconds and just go back to normal as if nothing happened. When I look in the task manager, the process just shows up for a breif second, thens goes away. Does anyone know what is happening? Please help, this is extremely frustrating.