XP boot failure (boot device not seen after o/c req'd cmos clear)

MSI 7071 mobo w P4 640 win XP home new install.
Was playing with overclocking options. Boot failed so cleared CMOS
After that started hanging up on boot "boot device not detected..."
So checked the BIOS aok
Ran UBCD seagate tools HDD OK
Repair console: chhdsk; mbrfix; bootcfg /rebuild; Fixboot

Didn't work so repair installed windows.
Problem recurred!!
tried again fixmbr etc etc
NOTHING WORKS
But interestingly if I leave the xp cd in the drive and allow the boot to proceed, (ie dont "click any key to boot from CD") XP boots from the HDD no problem. To my mind >> mbr or similar issue...have any of you guys you had a similar case?

Any suggestions? Or am I at the nuclear option: complete reinstall?

Much obliged if you can offer any help.
 
So I reinstalled XP from scratch formatting the drive Install went smoothly.
Took the disk out and rebooted. Guess what?
"boot drive failure insert boot disk...etc"
Now I'm really stumped.
 
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