I'm trying to delete old SVN files from directory tree. shutil.rmtree and os.unlink raise WindowsErrors, because the script doesn't have permissions to delete them. How can I get around that?

Here is the script:

# Delete all files of a certain type from a direcotry

import os
import shutil

dir = "c:\\"

verbosity = 0;

def printCleanMsg(dir_path):
    if verbosity:
        print "Cleaning %s\n" % dir_path

def cleandir(dir_path):
    printCleanMsg(dir_path)
    toDelete = []
    dirwalk = os.walk(dir_path)
    for root, dirs, files in dirwalk:
        printCleanMsg(root)
        toDelete.extend([root + os.sep + dir for dir in dirs if '.svn' == dir])
        toDelete.extend([root + os.sep + file for file in files if 'svn' in file])

    print "Items to be deleted:"
    for candidate in toDelete:
        print candidate
    print "Delete all %d items? [y|n]" % len(toDelete)

    choice = raw_input()

    if choice == 'y':
        deleted = 0
        for filedir in toDelete:
            if os.path.exists(filedir): # could have been deleted already by rmtree
                try:
                    if os.path.isdir(filedir):
                        shutil.rmtree(filedir)
                    else:
                        os.unlink(filedir)
                    deleted += 1
                except WindowsError:
                    print "WindowsError: Couldn't delete '%s'" % filedir

    print "\nDeleted %d/%d files." % (deleted, len(toDelete))
    exit()

if __name__ == "__main__":
    cleandir(dir)

Not a single file is able to be deleted. What am I doing wrong?