You were able to retrieve your files into Win7?

Please post back a screenshot of your maximized Disk management drive map and listings using Snipping Tool in Start menu to take a rectangular snip, save and attach with paper clip in reply box. We will give you the steps to delete D in Disk Mgmt to Extend C into that space.

As long as the System Active boot flags are all on C then you can go ahead if both partitions are Primary:
Partition or Volume - Delete D
Partition or Volume - Extend C

If you have programs that will only run in XP, try installing them to Win7 in XP SP2 Compatibility Mode first. Then if necessary try installling Virtual Player, Virtual Box, or VMWare freeware to install XP into to see how you like running XP virtually in a player for those programs. These are much better options than trying to Dual Boot XP at this late date. However if you must we can help you with that as well.