Can I safely delete the D partition?

[SystemJinx]

You could create one or more custom partitions while installing XP. More Windows XP install instructions can be found here. Just create a Windows partition that doesn't take up the entire drive. Use the remaining space for a second partition to store all your data.

Or if you want, just repartition the C drive while you install Windows, and do not erase the D drive. That way everything in the D drive is still there once Windows is up and running.