I've posted a piccy of a set of windows masks I can cut / post in my paint thread here as i've had quite a few requests for them.
Borders (black parts) are all pre cut - so you apply the entire window mask as one piece - then peel away borders to paint - simples
http://oople.com/forums/showthread.php?t=54175&page=13
Have a nosey - and get in touch of you want a set
Cheers.
Andy