Windows explorer association with .7z files problem

Hello. Today i *ACCIDENTALY* (yeah totally you idiot) changed the default program to open .7z files to windows explorer. Now, i cant changed back to winrar/7zip/anything else. When i go into properties there is no option for "default program to open with". If i right click the file and click "open with" then "set default program" and i click on winrar. It changes for a split secound and then...