If this is Access 2003, try viewing your forms with and without the
following option checked ...
From the Tools menu, select Options. In the Options dialog, select the
Forms/Reports tab. Find the 'Use Windows Themed Controls on Forms' option
down in the lower left corner of the dialog box.
Check this option to use rounded Windows XP-style command buttons, uncheck
it for square-cornered, Windows 95-style command buttons.
This option will also affect the appearance of other controls on your forms.
There is, as far as I know, no way to change the appearance of command
buttons independently of other controls. But if you don't like the result,
you can easily change it back again.