For Pietro (and anyone else who ends up with a similar problem), i managed to fix this issue by removing all of the php from the styles.php file (hence turning it into a plain css file)
This limits the flexibility of your cascading style sheets, however it is guaranteed to work!
Im not sure why this only effects IIS 6, but im sure there is a better solution for my "workaround"
General help
Problem with IIS6 and windows 2003 server
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