1) If you have another XP system lying around, make an XP startup disk. Insert a floppy disk into the floppy drive of the similar XP system, launch My Computer, right-click the floppy disk icon, and select the Format command from the context menu. Once the format is complete, close the box , double-click the drive C icon to access the root directory, and copy the three files- Boot.ini, NTLDR and Ntdetect.com to the floppy.
Now insert the floppy into the floppy drive on the problematic system and press [Ctrl][Alt][Delete] to reboot the computer. When you boot from the Windows startup disk, the computer will bypass the active partition and boot files on the hard disk and attempt to start Windows XP normally.
2) Did you try Last known good configuration, safe mode etc.?
3) Normally, the handiest thing one used to do was to install the XP recovery console on the hard drive itself. That way one doesnt need the cd in case something goes wrong.
4) I daresay if you look around or ask around, you may be able to get your hands on a 64 bit xp disk. But that UBCD you used, it would have fixmbr kind of options. Try repairing the mbr from that.