SPS webpage can detect you have installed the Office 2003 and run local dll to implement some SPS function, e.g. multi-file upload only works when you have office 2003 installed.

Integration with Office XP is gone.

You will get guys telling you that you can integrate with SPSv2 if you install a backwards compatible document library - but that's really just putting a bit of SPS 2001 on the server.

Believe me, check-in, check-out, which are themselves very basic, are not available from inside Office XP, or even from the context menu in Windows Explorer.

The ONLY option you have is to use the web interface to check-in or check-out.