Actually, Service Pack 1 enables large drive support. If you installed
Windows with an original Windows XP CD (without a Service Pack
integrated), you will only be able to make a partition of 127 GB in
size. You have two choices. You can either slipstream at least Service
Pack 1 into your XP CD and do a clean install allowing you to make one
large drive, or you can make a second partition with the extra 22 GB of
space you are missing.
To do this, right click on My Computer, click Manage, click Disk
Management. Right click on the Unallocated space, then partition it and
format it and assign it a drive letter. This will make it appear as two
hard drives on the system.
There is a way to merge the two, but this requires either Windows XP
Professional or you must purchase a 3rd Party software program such as
Partition Magic.