No, you misunderstand...
Boot to your Snow Leopard DVD.
Run Disk Utility, from the
Utilities menu - ignore any message that says you can't install (you're not installing anything, just running Disk Utility)
Click the hard drive (the line with manufacturer's info), then click the Partition tab. Change the partition drop down to "1 Partition" and click the Apply button. Double check that the partition is GUID.
When that completes, it should let you install.
This is all assuming that you have a commercial Snow Leopard installer DVD - and NOT a DVD that shipped with a different Mac model. That one would complain that "software can't be installed on this computer", but that would, in no way, affect booting to that DVD and running Disk Utility.
BTW, the link that oldhifi posted is the Apple Store, where you can purchase a real Snow Leopard DVD - which will work if you can't use the DVD that you have now.
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
Only $20, and they'll ship it right out to you...