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PC had been working for years, now would not boot.
Starts to boot then stops with a blue screen and the message
UNMOUNTABLE_BOOT_VOLUME

I tried booting from a Hard Disk diagnostic CD and ran the quick and long tests both passed OK.

Microsoft have an article
http://support.microsoft.com/kb/297185

So I booted off the Windows XP CD and typed R to start the recovery console at the first screen requesting user interaction,

I selected 1 as there is only one windows on the PC.
entered the administrator password
and ran chkdsk /r
This took some 45 minutes on my 20 Gb drive

THE CHKDSK completed OK reporting that it had repaired errors.

When I restarted the PC it started normally.

Laptop has the UNMOUNTABLE_BOOT_VOLUME error
I have a stop mesassage at the bottom of the screen.
stop: 0x000000ED

I place a XP Operating System disk in CD drive
Power on laptop Press F12 and tell the laptop to boot from CD
Press any key to boot from the CD when this message appears
When prompted hit the R key to go to the R=Repair.
when get to the dos prompt
Run the Check Disk Program with the Repair Parameter i.e. type
CHKDSK C: /R