|The disk error could be from trying to boot from a partition that does not contain an installed Operating System.|
I suspect your friend simply placed a back-up copy of XP on the new partition, rather than a second installation of XP. If it were a Second install of XP, I believe you would have a Boot.ini file that would present the dual Boot options on start-up.
Are you able to boot to WindowsXP from C: to either Normal or Safe Mode? If so, you can use the backup on 2nd Partition to do an over-the-top install of Winxp on C:.
Without a Floppy or CD your choices are limited, actually, Floppy is mostly useless on XP, for what you need. You cannot even run HDD software without one or the other.
Also, it would be helpful to know what problem you are having with WinXP on C:
If C: is producing the Disk Error, you may be dead-in-the-water with this HDD, unless you can install it in another Computer (External USB Drive for Laptop HDD)where you can run the HDD Software, and copy files from Partition x to C:
There is nothing to learn from someone who already agrees with you.