Pennyboy,

You don't tell what you want, and why you want it, so just a general answer.

Your options are:
1. Add a second harddisk. That will 'automatically' (if installed correctly, but we can provide links teaching you how to do it) become a second drive letter and give you more disc space for storage of your data. If you've got 80 Gb now, and buy an additional 120 Gb, you've got 200 on 2 physical disks.
2. Make a new partition (drive letter) on your current hard disk. If you've got a c:-drive of 80 Gb now, you change this in, say, a c:-drive of 30 Gb and a d:-drive of 50 Gb. This can't be done using (only) Windows XP, you need a third party program like Partition Magic to do this.

In both the above cases, there's no need to do anything with the Windows XP installation.

Tell more if you want something else.

Kees