Vista has an annoying quirk that protects all files in the Program Files directory. So, if you try to checkout into a C:\Program Files\SCAR 3.13 directory, it will do crazy stuff like give you errors or copy the files to some other weird folder they don't belong.
The easy solution is to install SCAR OUTSIDE the Program Files directory!
I got the Could Not Move Plugins error after installing to C:\Program Files\SCAR 3.13. After I uninstalled there and re-installed in C:\SCAR 3.13 it worked perfectly.
Try it out if you're using the Microsoft VISTA operating system and having problems.