One day morning I tried to reboot my PC but message popped up saying can't find boot device.

   The boot device is usually the C drive.
   There is an order to which devices are tried for boot during startup.  Devices that are on the list before C can cause the error and bring the boot process to a halt.
   If you have a floppy drive, it usually is tried first.  If there is an unbootable diskette in the diskette drive, you will get that message.  Take the floppy diskette out and press a key and it should continue starting up.
   The CD drive is also often in the boot list but usually if an unbootable CD is in the CD drive, it will be skipped and will not cause that error.
   If you don't have a diskette in the floppy drive or any other device in the boot list which is causing the error, the problem is probably with the C drive.  Either your drives got switched and so your computer is trying to boot from a different drive that is not bootable or your C drive has become unbootable.  The drive has a portion at the beginning of it that is the boot sector.  If the boot sector is not there or has been corrupted, you will get that error.  Normal usage of the computer should not mess up the boot sector, but if you were doing things that you should not have done, well.....
   If C is the problem, you will need a boot diskette or CD to start your computer and fix the problem.  The boot sector can be repaired in DOS by typing fdisk /mbr.  You can lose all of the data on the drive by doing this, so please do not use that unless that is the problem and you have backed up the drive.

Try to go to the BIOS and check all options at Boot ; decide if your HD is the second boot device (first shoud be floppy , or CD -Rom drive) ; if that does not work , boot from the Windows CD and try to REPAIR the former installation ; for this do not ask for repair at the first prompt , instead click INSTALL ; you will then be proposed some choices ; there you may find the repair option ; in any case , give more indications on your system . best regards OLIVIER GEBUHRER

I've had trhis problem many times. If you use you XP set-up disk then chose R for repair. Check number 1 for the drive and then type help at the dos prompt. Scroll to the bottom ant then type fixmbr and you won't lose any of your information.