Ashish,
Your title and your question made reference to two different drives and 2 different systems.
If you have installed a new harddrive and installed WIN7 on it you should still be able to read any and all the data on D Drive, and you can copy,move,delete any information you want off D drive as you see fit to create space. as C drive is your primary and D is your slave.
If you or someone installed WIN7 on a partition of the same Drive and you are trying to create room by deleting XP you will need to be careful.
now a days Hard drives are not that expensive you may want to look into just getting a second internal drive if you can.
Don