Weird that the install of a new IDE Device could corrupt a HDD Sector. Possibly if you run the recovery console from the XP CD you can rebuild the boot sector which may allow successful bootup.

Insert the XP Disc
Set first boot device as CDROM in Bios
Reset
XP will copy files across for the Recovery Console
Run the Console
Select the installation to repair
Enter password if Applicable
Type: FIXBOOT
Answer Y
Hopefully it boots up

I meant to add as well:
At the point where you will type FIXBOOT you can try this:
COPY D:\i386\NTLDR C:
COPY D:\i386\NTDETECT.COM C:

This will replace your crucial 2 boot files - although it would seem the whole boot sector needs re-writing.