repairing compaq presario M2000 series

I have a Compaq Presario M2000 series (M2105US). It works well and I
have had no problems with it other than that when I shut it down I am
unable to reboot without reinstalling Windows XP using the original
disk. When I shut down and later turn it on, I get an error message that
says "internal boot error" (from what I remember) I have not been able
to permanently fix the problem, so I now keep my computer on all of the
time and avoid shutting down unless absolutely necessary. I have no
difficulty operating the computer after reinstalling the OS, and it
works properly until I shut down. Then I must repeat the OS installation
over again. But unless I shut down the computer, there is no problem. It
seems to take longer for programs to load than previously, but nothing
major aside from the problem with shutdown.