We have a copy of ArcGIS 10 that we are preparing to use to upgrade our existing ArcGIS 9.3 installation.
Should I uninstall ArcGIS 9.3 first?
Or do I install ArcGIS 10 over it?
Are there any best practises or guidelines I should be aware of?
You must uninstall a previous version of ArcGIS software before installing a new one. In fact, the install programs will require it. In addition, I suggest that if you have any Esri or 3rd party tools that you uninstall them first, before uninstalling ArcGIS 9.3.
A related question was asked earlier, How do I cleanly uninstall ArcGIS 9.3.1 extensions after ArcGIS 9.3.1 is uninstalled?
ArcGIS 10's default installation location is different from 9.3.1, so that helps minimize some possible conflicts. Here are the steps that I would follow:
You also might want to look through the ArcGIS installation forum.
Std Disclaimer: I am an Esri employee, but not on the install team.
Best practice is to generally remove older versions of software before installing new ones unless there is a chance of interfering with complicated custom settings, although these may be overwritten anyways.
That being said, you can install Arc10 over an existing 9.3 version and it will work.