Unable to open .exe files

Vista won't load .exe files! And more problems...

Ok, so all of a sudden, my computer goes berserk and I get all these pop-ups and messages saying that my computer is infected. Then it continues to say that I can't use the computer unless I delete the Trojans. I press the Remove Virus button and it takes me to a page on IE that says I have to pay for this Windows Vista Security package to remove the viruses. I ignore that, and proceed to download Avast! the great free anti-virus program that so many people have told me about.

Well, I tried to open Avast! but Windows kept saying that that file was infected. So I shut down several times, and I still had the problem. So I gave up, went to bed, hoping that the problem would be fixed tomorrow.

Next day: I opened Windows, and I didn't have any of those pop-ups. But, I couldn't open ANYTHING. I tried to open Firefox, and something popped up saying something like "Windows cannot find the correct program to open firefox.exe, choose a program from the list" and all that was on the list was Internet Explorer.

When I tried to open Microsoft Word, it said that it couldn't find the correct program, and told me to choose frmo the listed programs, which was just Internet Explorer.

Same thing happened with Google Chrome, Avast!, and Notepad.... All of them were prompting me to open them with a different kind of program.

Thing is, when I try to open IE itself, it prompts me to open it with the correct program-opener, and lists Internet explorer.

I didn't get those pop-ups, or messages telling me I had viruses anymore...

Please help, as soon as possible.