You can compare 2 different files using SVN Diff! Just go into Windows Explorer, select 2 files (ctrl click) and right click on one of them. You’ll get a context menu like this:

Just click SVN Diff (or you might see TortoiseSVN > Diff instead).

You’ll get a nice side-by-side comparison of the differences in the two files.

(P.S. This was done using Tortoise 1.6.6)