UNMOUNTABLE BOOT VOLUME Error Message

I have a Dell Dimension 8400 Desktop pc...when I switch it on the Windows XP screen shows with the progressing bar showing.  After an abnormal time of trying to load I get this error message:

"A problem has been detected and windows has been shut down to prevent damage to your computer.

UNMOUNTABLE BOOT VOLUME

***Stop: 0x000000ED (0X8676F030, OX0000006, 0X00000000, 0X00000000)"

The error message also suggests these actions.

Disable memory such as caching or shadowing...If you need to use safe mode to remove or detect components, restart computer, press F8 to select advanced set up...Then press Safe Mode…

When I pressed F8 my screen fills up with various messages … and it never gets to Safe Mode. 

The problem started when I was running Google Earth and was zooming down.  It hung up in some kind of mode and it would not stop zooming in.  I tried Control-Alt-Delete to get to the end task screen but the computer would not respond so I had to shut it down.  When I tried to re-start I got the error message above.

Any help provided would be very much appreciated.  Thanks.