How to fix Windows Executable (.exe) problems?

Ok i have a bit of a predicament... I recently had gotten almost taken over by a computer hacker. But I ended up disconnecting everything and erasing mostly all the files the guy used to take me over. But there was this one file left over, a .EXE file, that I couldn't delete, edit permissions for, anything. so i changed what program is used to open it, so that it doesn't actually work. I changed it to Windows Media Center.... and so all other .EXE files changed with it, like iTunes, FL Studios 9 etc. So now i can't really do anything until i fix this problem, despite multiple attempts to change the program opening such and such. Please help!