your xp partition is likely still there. The drive labels (C:\) are relative to each OS. XP would say that it's OS drive is C, and vista is D perhaps, while Vista would inform you of the reverse. Since applications do not (in general) work across OSs, the naming convention is of little concern
A key thing to note is that it lists the drive as 18GB which is a good sign that the other partition is there, just potentially invisible.
*Or are you able to see the 2nd partition?
there are a couple things i would suggest:
http://www.syschat.com/dual-boot-vis...6.html?garpg=4
steps 3 and 4 of this tutorial.