Set the environment variable TCL_LIBRARY in your activate script. On Windows (Python 2.7 with Tcl 8.5), just add this line to Scripts\activate.bat:
set "TCL_LIBRARY=C:\Python27\tcl\tcl8.5"
@Jasper van den Bosch’s edit: On Ubuntu, the modification to the script activate is the following:
TK_LIBRARY=/usr/lib/python2.7/lib-tk:/usr/lib/python2.7/site-packages/PIL:/usr/lib
TKPATH=/usr/lib/python2.7/lib-tk:/usr/lib/python2.7/site-packages/PIL:/usr/lib
TCL_LIBRARY=/usr/lib
export TCL_LIBRARY TK_LIBRARY TKPATH
The reference of this can be found on this question on askubuntu