you should be able to boot to a Recovery Console, log into it (see http://support.microsoft.com/default.aspx?scid=kb;EN-US;q308402 if you have problems with this) and then run a chkdsk /r.  This will reset the $logfile and allow you to enter the system.  I would suggest getting the patch off of Windows Update for http://support.microsoft.com/default.aspx?scid=kb;EN-US;q315403 when the system is up and running again

If the error message is "STOP 0x000000ED UNMOUNTABLE_BOOT_VOLUME" Error Message When You Restart Your Computer or Upgrade to Windows XP see: