Have you tried booting to safe mode? What is the make, model, any service
packs installed? What is the exact sequence of events when you switch on
(including verbatim error messages)?
0x000000ED: UNMOUNTABLE_BOOT_VOLUME
The kernel mode I/O subsystem attempted to mount the boot volume and it failed. This
error also might occur during an upgrade to Win XP on systems that use higher
throughput ATA disks or controllers with incorrect cabling. In some cases, your
system might appear to work normally after you restart. http://support.microsoft.com/?kbid=297185&sd=RMVP