Boot from the XP CD and go into Recovery mode:
1. Press any key to boot from CD
2. Let the setup start from CD
3. Wait for all the "Setup is loading files..." messages to scroll along the bottom of the screen
4. At the series of options under Weolcome to Setup, select the second option "To repair a Windows XP installation..." by prssing the letter R key
5. When it says "Which Windows installation would you like to log onto" select the number that corresponds to your system - normally you type number 1 for C: drive, but any variations will be listed above the option to select. Type in the number for your drive/pratition then press Enter
6. Type in the Aministrator password to access the drive then press Enter. If setup did not or has not completed then you may be able to just press the Enter key since there may not be an admin password.
7. type this command:
then press Enter
You should get a prompt about are you sure and then a message saying it succeeded.
Restart the system by typing Exit.
FDISK/mbr is not a supported option in the Windows XP recovery console. The fixmbr command overwrites the old MBR with a clean Master Boot Record.
If the worst comes to the worst and it still does not complete the setup you may have to use your laptop revovery disks and then run the upgrade again.
I'll assume that your data files were thoroughly backed up prior to running the upgrade ?