As far as I know, when you put in the upgrade disk for Windows 7 into your XP machine, the install process will look normal.
What this means is that when you get to partitioning screen where you choose where to put Windows 7, you can just simply choose the hard drive that currently houses XP. All of the XP files will be put in a Windows.old file and your data on the other drive should still be in tact.
This is of course assuming that your C: drive is strictly for Windows and your D: drive is strictly for Data.