Make python 3.3 the default one and not 2.7

[D. Xenakis]

Hi there, i installed python 2.7 (windows 32bit version) from http://www.enthought.com/products/epd_free.php and after that i installed official  3.3 version too. So now i got two python folders like this.. c:/Python27 and c:/Python33 . 
My problem is that when im trying to execute a .py file, the 2.7 version interpreter launces. In addition if i right click and select Edit with IDLE, again version 2.7 interpreter opens. How can i make python 3.3 the default one, as i need this cause i have PyQt4 installed there and so it does not work with 2.7 .
Thx in advance.