The extension rar is used for a type of archive named RAR. Simply put, it is an alternative ZIP, better than ZIP IMO.
The official software to utilise RAR can be found at the
WinRAR website. WinZIP claims to be able to use RAR archives, but often fails, and won't let you know, so there will be files that are in the archive that you wouldn't know existed if you use WinZIP to open RAR archives.
In the deviation linked by Methos2, the RAR download contains Vista.wba, which is for use with WindowsBlinds (what is the file type called? I've only use WindowsBlinds for a bit)
Update
After a bit of digging around, it seems to me that there was a WindowsBlinds edition of VistaXP, and a Microsoft Visual Style edition. The latest WindowsBlinds edition was v3.0 and the latest Microsoft Visual Style edition was v2.0.
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