windows preview bug fix

[Idan Pazi]

I have sys.flags.utf8_mode = 0.
I assume everything is utf8, but I don't think the problem is about finding the right encoding.
win32file.ReadFile returns bytes, which cannot be added (+) to a string (see data = "" at line 162). 
Assume that on some cases it returns a string, then this patch won't change a thing.

BTW, I think it is possible to use only python Popen on windows without using pywin32 library.
would you consider to look at a patch doing that?

Thank you, Idan