From Birnam Designs Wiki

Some zip/rar/etc. applications, when installed, will overwrite Vista's default handling of zip files as simple "Compressed Folders". To restore the normal behavior:

Click Start, type regedit.exe and press Enter. Navigate to the following branch:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.zip

Delete the sub-key named UserChoice

Close regedit.exe