The current file-size of ESO is about 45GB. I'm not sure how much of that is still the original stuff that's on the disc, so you may still have a pretty large download, but the disc is probably the best way to go.

Alternatively, you could try copying the data files across, but since it's Mac to PC I have no idea whether that would work. (It does work for PC to PC, as I've done it before.)

If you want to try that route, download the launcher from the site, start the installation from the launcher, and then cancel the installation when the large download starts. That should create the necessary folders, as well as a few patch data files which you'll need to delete so the folders are completely empty. Then disable the launcher's automatic updating and close it. After that, you need to copy the contents of "depot" and "vo_en" from wherever they are on the Mac to their new destination on PC (I would leave "game" alone, as I think that's where it keeps the data for the different clients). When the copy is complete, go into depot and vo_en and delete the files depot.version and vo_en.version. Finally, reopen the launcher; hopefully at this point it will say "Update" instead of "Install". You then need to run Repair on the launcher (not Update) so the missing .version files and client files can be downloaded.