May be you can find the ".Grub" folder and delete it.

I solve the similar problem like that: boot from a boot disk, find the .Grub folder in C:\, I delete it, reboot, then I see XP welecom window.
But Maybe your .Grub folder is not installed in C:\. May be you installed it in a Linux partition. If your .Grub is not in C:\, you have to execute fixmbr command in C:\.

By the way, if you boot from other boot disk, may be the C:\ is not the disk you want. Take it easy, just find where the Windows folder, and that disk it is.