If you start
/Applications/Utilities/Boot Camp Assistant, you should be able to select your Windows partition for removal. Your Mac OS X partition will be automatically resized to use the then unused disk space. This will not remove your Mac OS X partition. This is how I did it a while ago, IIRC.
I don't have two partitions on my Macs at the moment, but when you select the disk in
/Applications/Utilities/Disk Utility, and select the Partition tab, can't you select the Boot Camp/Windows partition, click the
– button to remove it, and increase the size of your Mac partition afterwards? It should look a bit like this:
If you cannot select the Windows partition for formatting, make sure it's not configured as the startup disk.