HmmmYou should have selected the remaining space while installing XP to create a second partition there That way it would show up as D: or whatever in My Computer.
Anyway - you can do the following :-
1] In XP - Control Panels
2] Performance and Maintenance
3] Administrative Tools
4] Computer Management
6] Disk management (Local)
7] And at the bottom of this window you will find the necessary tools to partition the remaining space
Please let us know if this workd...