Says "computer not authorized", but it is

Several apps have been taken off my iPhone and iPad. When I try to put them back on I get a message: "You are not authorized for them on this computer." I've followed the instructions to authorize the computer and it tells me: "This computer is already authorized." I've tried deauthorizing and reauthorizing and get the same results. I've deleted the SC folder as instructed and this didn't work. I've also changed User Account
Control settings and that didn't work either.
I have Windows PC running Windows Vista.
Any suggestions?