But as you have a Dual Core CPU it can use either a 32 or 64 Bit OS. I'm assuming here that you mean the 32 Bit version of XP when you say that you are using XP as well.
Now to install another OS you need free space on the HDD. Currently you will have the HDD partitioned to having all available to XP but as you want to install another OS you need to free up some space and then install the new OS to the Free Space. Until you free up some space by either shrinking the partition size or reinstalling the OS without using the entire partition you can not hope to install anything other than the OS you are currently running.
As things stand if you wanted to install a 64 Bit version of XP you can not do this either though while you can finish the install you will constantly be warned that you shouldn't attempt to install 2 versions of Windows XP to the same partition and when you finish neither will work.
Col