Python interprets a \t in a string as a tab character; hence, "D:\testfolder" will print out with a tab between the : and the e, as you noticed. If you want an actual backslash, you need to escape the backslash by entering it as \\:
>>> x = "D:\\testfolder"
>>> print x
D:\testfolder
However, for cross-platform compatibility, you should probably use os.path.join. I think that Python on Windows will automatically handle forward slashes (/) properly, too.
