The putative 5.1 mod we're apparently using is
here.
Note that WinRAR, WinACE, and PKZip (the three most common / popular programs I know that handle .rar files on Windoze) are all
licensed shareware products that require you to purchase a license after the trial period, while WinZip (which has the same license requirement and fee) doesn't handle .rar files.
The only one I know of for Windows that
doesn't have a moral if not a binding legal requirement to purchase it is tugzip, which while somewhat clunkier to use is at least free, found at
Tugzip's page.