When you open Windows Explorer from Windows 7, you can click the Computer icon in the left hand pane to bring up your partitions. I'm assuming you've done a single partition per drive, so you should see one that has your Windows 7 install (probably
, and another that has your Windows XP install. It might not necessarily be the D: drive, depending on what kind of non-disk drives you have installed, but it will still be listed in the Hard Disk Drives section.
From the explorer, right-click on the secondary drive. Toward the bottom of the context menu, you should see an option that says "Format...". Choose that option. This is what I see when I do so:
I would strongly recommend leaving the drop-down menu options as they are, unless something other than NTFS is selected as your file system. Unless you plan to dual boot with some flavor of Linux, NTFS is going to server you better. You can specify whatever you want in the Volume label text box - I used "Files" for my files drive.
The last piece of advice I would give you is to uncheck the Quick Format box. Doing a full format will do the extra step of checking each sector on your physical platters for hardware error. I get the impression that this is a bit of an older disk, so you definitely want it to map out the bad sectors so it doesn't try writing to them.
After all that, just hit Start. Windows is smart enough to not let you format the boot drive for the particular OS you're using, so you shouldn't be able to format your Windows 7 drive anyway. When it's finished, your second drive should be ready for writing.
If you do have a tangled mess of partitions to sort through, I would recommend using some sort of partition management software over the Windows boot disk. MiniTool Partition Wizard Home Edition is pretty damn good for a free tool if you need it, and it's way more convenient than using the Windows tools. Just be careful you don't blow away something you need.