Yes. You can Boot the XP CD to run an XP Repair Install
Once it repairs and hopefully claims C for XP, follow Steps 13-17 in Method Two of this tutorial to add Windows 7 to the Dual Boot.
If this fails, boot the Windows 7 DVD to mark 100mb partition Active: Partition - Mark as Active (Method Two)
. Then from DVD Repair console run Startup Repair - Run 3 Separate Times
until Windows 7 starts. Install EasyBCD 2.02 to Add XP.
The only other thing that I can think of is to mark XP partition Active before running XP Repair install to try to force repair to write XP System boot files to XP, thereby possibly claiming C. Then boot Windows 7 DVD to mark 100mb Active, Repair Windows 7, add XP using EasyBCD.
If neither of these work you can reinstall using Method One for the easiest Dual Booting method without possible interference by the 100mb SysReserved partition.