>>> import _tkinter
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: DLL load failed: %1 is not a valid Win32 application.

I 'upgraded' to python 2.6.5 by downloading the windows 64 bit installer.  I'm running windows 7 64 bit.  Everything worked great in 2.6.4 by the way.  I had a similar problem with numpy,  but reinstalling it from this site seemed to fix it:
http://www.lfd.uci.edu/~gohlke/pythonlibs/

I can only speculate what happened.  If I knew what DLL failed to load I could easily check to see if there were dependency issues or 64/32 bit issues.  Perhaps the old dll is still around? If so then there is an issue with the installer I think.

No doubt the issues extends beyond just Tkinter.