Your saying your old C: partition. You can only have one partition at a time with the C: label. If you installed one instance of XP on C: and put another on C:, then that means you either did a repair install of your first installation or installed over it since you said you formatted it.
During the installation, you can partition chunks of the hard drive space for partitions, but up until the release of Vista, you could not format other partitions using the installation media other then the one you are installing Windows on. So it can be quite easy for a beginner to dual-booting to accidently allocate the whole hard drive for a Windows instance rather then partitioning it out for each of the systems you plan to install.
Before you even begin to install, you should consider the desired layout of your system's partitions and thier sizes. Then when performing the first install, go ahead and create the partitions, but make sure to specify to setup the amount you wish to allocate for each partition. Choosing the default well allocate the entire remainder of free space on the disk, which is not what you want since you need to install each OS on its own partittion.