Recently, I upgraded Visual Studio and somehow in the process when I went to use SVN I started to get '%1 is not a valid win32 application' displayed whenever I tried to view the history of a file, or, do a diff. After a quick Google, I couldn't find anything relevant so I thought I'd quickly share the resolution.
The issue was down to the custom diff tool I had Tortoise set to use. In my case WinMerge. I hadn't uninstalled WinMerge, or changed anything. Reverting Tortoise to use the default editor got it back up and running.
On a folder, right-click to bring the Windows context menu up and select 'TortoiseSVN' -> 'Settings'
open the 'Diff Viewer' tab. In my instance, I had the 'External' option selected with WinMerge. After I changed this to ToroiseMerge, the error stopped.