Visual Studio and Diff tools problems

On my laptop, I have Visual Studio 2010 set in my Unity preferences as default script editor, yet when I double click a file, it always opens in UniSciTE. On my desktop machine, it opens every time in VS 2010. The differences between the two? on the Laptop, my Unity projects are mostly all on an external drive, but this happens even if I open a Unity project from one of the partitions on my main drive too. Is this a permissions-related issue, security sandbox or similar on the part of VS & Windows? It's annoying, and I can live with opening the sln separately, but would be nice to fix.

Both machines are Windows 7 64bit, both set to the same UAC setting of warn when programs try to make changes to my computer (but don't dim my desktop), laptop is Win 7 64 Professional, desktop is Ultimate, but I'm scratching head for any other relevant differences...

On a related note, I have Tortoise SVN and Visual SVN installed on my laptop, and for a long time Tortoise Merge would work great for the diff tool for my asset server connection. I'm not sure why, but I now have the message that "No supported Asset Server diff tools were found". I can't think of what I might have done to break the connection, but I can't select a tool by hand. Is this setting picked up from somewhere else? Should I try editing my environment variables or am I looking in the wrong place for a solution?

Thanks all in advance...

Chris