XP Power up problem

XP will not power up unless I hot boot after the first boot attempt fails after mup.sys. Every time I hot boot it starts perfectly.
BIOS is set to look for the hard drive then cdrom on boot up.
I have tried updating all XP critical updates; repairing XP via the installation disk (not the repair centre function but the reinstal function); boot logging (I cant see any problem on the log); moving the boot sector of the hard drive; defragmenting C; Norton one button system check; Full virus scan with up to date Norton Antivirus.
What am i missing?
Ian Horton