The reason you're having problems is that the version of Boot Camp you used to set up your Windows partition "expired" and you can't (easily) use it to remove that partition. You could set your system clock back to some time in 2005 and then use the old Boot Camp Utility to delete the partition-unless the partition table has become corrupt for one reason or another (repeated tries to delete that partition, Windows itself, etc.).
In that case, use Carbon Copy Cloner or other, similar tool to back up your OS X partition. Now boot off your Leopard DVD and use Disk Utility (under "Tools) to repartition it to a single HFS+ (OS X native) partition. Then restore your OS X backup. A pain in the rear, no doubt about it, but that will fix the issue. The reason you couldn't mess with the startup disk before was that you were using Disk Utility from OS X, but it can't do that because you're running the OS from that disk... Using Disk Utility from the Leopard DVD will do the trick.