Create a new PST in OL 2000: click File>New>Personal Folders File, browse to
the folder where you'd like it (or accept the default), give it a file name
(or accept the default).  Click OK.  Give it a display name (or accept the
default).  Click OK.

Right-click your calendar and drag it to the new PST you added.  Choose
Copy.  When the copy completes, close the new PST with right-click>Close.
Close Outlook.

Transfer the PST to the OL 2003 machine.  Put it in any folder you like that
does not contain a PST of the same name.  Remove the read-only attribute, if
necessary.  Open Outlook.  Click File>Open>Outlook Data File.  Browse to the
PST, select it, and click OK.  You'll now have access to the calendar you
put in it.  Open that calendar, set your view to a list view like By
Category, select one item, then select them all with CTRL-A.  RIght-click
and drag the selection to your main calendar.  Choose Copy from the context
menu when you release the mouse button.  When the copy completes, close the
added PST with right-click>Close.