Opening Programs w/out 'open with' appearing

Hello,

I use Vista and everything was dandy until I got a virus. After getting rid of this virus (the vista internet security virus) I found that all my programs won't run as they used to. If I click on an icon for firefox or a media player the 'open with' box turns up as if it doesn't know how to run the program. In order to use anything I have to right click the icon and run as administrator. And I have to do this every single time. Any programs that started up automatically when turning my laptop on don't start up automatically anymore. If i type 'regedit' in the search bar at start a box comes up and tells me that the application can't be found. Same thing happens when trying to use a program from my Dell Dock.

If someone could help me I would very much appreciate it.

Cheers in advance.