The advantage of having the OS on one partition and your other applications and files on another is that if you have to reinstall the OS you will only need to reformat the partition with the OS, the applications and files that are on the other partition will not be effected.
There are applications that can be used to add a partition. I have seen GParted
recommended for this, but have not used it myself.
As has been suggested, if you have the Windows installation CD you can always reformat the hdd and add the second partition. You will need the other CDs with the drivers for the motherboard and the other applications that aren't part of Windows. Or...you could just live with the one large partition. Just make sure that you regularly back up your important files and save the installation setups for any downloaded applications to a removable media,CDs, DVDs, flash drives, backup hdd. .