IIS Mod-Rewrite and Imagecache

I run Drupal 5.7 on IIS 6.0 with IIS Mod-Rewrite Pro v3.5

I have just added this line at the beginning of index.php

$_SERVER['REQUEST_URI'] = $_SERVER['HTTP_X_ORIGINAL_URL'];

Clean URLs are working OK.

I have trouble with imagecache, that does not work.
It seems to be the .htaccess file in the Drupal files directory that cause trouble. Changing this .htaccess content does not change anything.

If I remove it, imagecache works OK. But Drupal recreates .htaccess file it each time file_check_directory is called. As temporary workaround, I have commented this section in file.inc, but I think this is not the right solution.

Can someone give me more details about IIS Mod-Rewrite and .htaccess configuration ?

Frederic