I wrote a Tkinter application that runs fine with Python 2.6.5 and 2.6.6 on a Windows 7 (32-bit version) machine. The same app fails with 2.7.1. Below is the debug output:

c:\ISS\ledmatrix>led-display.py
Traceback (most recent call last):
  File "ledmatrix.py", line 2263, in <module>
    LEDMatrix()
  File "ledmatrix.py", line 1427, in LEDMatrix
    root = Tk()
  File "C:\Python26\lib\lib-tk\Tkinter.py", line 1643, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: Can't find a usable init.tcl in the following directories:
    {C:\Python27\tcl\tcl8.5} C:/Python26/lib/tcl8.5 C:/lib/tcl8.5 C:/lib/tcl8.5 C:/library C:/library C:/tcl8.5.8/library C:/tcl8.5.8/library

C:/Python27/tcl/tcl8.5/init.tcl: version conflict for package "Tcl": have 8.5.8, need exactly 8.5.2
version conflict for package "Tcl": have 8.5.8, need exactly 8.5.2
    while executing
"package require -exact Tcl 8.5.2"
    (file "C:/Python27/tcl/tcl8.5/init.tcl" line 20)
    invoked from within
"source C:/Python27/tcl/tcl8.5/init.tcl"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 [list source $tclfile]"


This probably means that Tcl wasn't installed properly.