According the the Grub Manual, error 15 is:
File not found
This error is returned if the specified file name cannot be found, but everything else (like the disk/partition info) is OK.
This manual can be found at:
Let's go back to your original post. You updated system.map? What about grub.conf? From my understanding of Grub, that should have been the first file (and probably the only one) that you needed to update.
Where you seem to be at now: Nothing works.
I don't think you have messed up your MBR--both Windows and the Linux boot loaders installed there just fine. Grub getting to Stage 1.5 means that it has left the MBR.
Since you want to use Grub, how about we get Linux up first, then work on XP?
Use your boot disk and start Linux as root. When it is up and running, in a terminal window, type:
That should should get Linux booting back to your original kernel. The Grub manual will tell you how to add your new kernel.
In a perfect world (I can dream, can't I?), this will also fix the problem with XP booting. My guess is it won't, so in your next post, give us your grub.conf file. If I'm wrong about system.map and you are supposed to alter it, also post that.