I'm not a Windows guru, so I'm not a big help here, but it should be trivial because (in fact) no installation is actually necessary. All you need to do is download the distribution and unzip the archive. Then you can launch exiftool like this:

Perhaps this description from a more Windows-savvy user will help. According to him, you can use either WinZIP or pkzip to unzip the distribution archive.

I hope this helps.