how do i save to unpartitioned space?

Excuse me if this question has been answered but basically i just set up a new hard drive with a xp home sp3 and when it asked me to make a partition i didnt really know much about it so i read some other forums that said i should make a 15GB partition for the OS, (so i did). Well now I'm trying to install a game and I've come to realize my C: Drive is only 15GB (Now i get what i made earlier). But where is the other 145ish GB of space in my hard drive and how do I install my game onto it. Tyvm for any help you can offer.

Hey back again i did some research on youtube and found exactly what i was looking for (Shocker! :0). But to answer my own question for any1 else who comes across this forum. On XP you need to go to start, right click "My Computer", and select manage. Select Disk Management (under Storage) on the left side and at the bottom you will see your disk drive with the partitioned space and the non-partitioned space (shown as unallocated). Select the unallocated space then right click it and select "Partition", or something similiar to that. Then just go through the screen and you will be able to choose what you want the drive letter to be and what not.