Following a blog, I created a batch file, wm.bat:
\"d:\\svnroot\\external\\winmerge\\WinMerge.exe\" /B /WAIT \"d:\\svnroot\\external\\winmerge\\WinMergeU.exe
Meld works really great for me. After installing, I went to the Subversion config file, uncommented the diff-cmd line and set it to the path when meld.exe is. In my case:
[helpers]
### ...
### Set diff-cmd to the absolute path of your 'diff' program.
### This will override the compile-time default, which is to use
### Subversion's internal diff implementation.
diff-cmd = D:\SOFT\Meld\meld\meld.exe