It is not recommended that you use the power button to turn off you PC
on a regular basis. Windows saves a number of files during the
shutdown process. Using the power button prevents those saves. I have
had to rebuild a number of PCs for friends that used the power button
to shut down. Eventually Windows became corrupted and would not start.
Unless of course you have set Power Options Set to "shutdown the
computer", in which case Windows XP will run the "shutdown routine" just
like you had picked "shutdown" from the menu.