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I actually had a similar problem just the other day except W7 wouldn't recognize my SuperDrive via USB. Anyhow, I ended up having to make an iso image of my snow leopard disc, download daemon tools lite, and copy both from the Mac partition to the bootcamp partition. After installing daemon tools and mounting the iso bootcamp popped right up and installed.
Since you don't have a SL disc on hand, the next best thing for you to do would be to see if you can get your hands on a SL iso image, I'll leave it up to you as to how you go about doing so 😉