So, here is a workaround in case anybody needs it:
1. Download and install the Microsoft Application Compatibility Toolkit 5.0 (from the Microsoft website)
2. Start->Programs->Microsoft Application Compatibility Toolkit 5.0->Compatibility Administrator
3. Click the "Fix" button at the top (if it is greyed out you may need to make a new custom database first and select it.
4. A create new application fix wizard will appear.
5. Wizard - Program Information screen: Name of application and name of vendor don't matter too much, put anything there. For program file location, browse to the GVI setup program on the CD-ROM. Click Next.
6. Wizard - Compatibility Modes screen: By default "Microsoft Windows 95" will be selected. Change this to "None" and click "Next"
7. Wizard - Compatibility Fixes screen: Scroll down and check the check-box next to "WinXPSP2VersionLie". Click the "Test Run" button (DO NOT CLICK NEXT).
8. Test run window: Ensure the path to the program is correct (should be something like e:\setup.exe). Command line can be blank. Click the OK button.
9. The GVI installer will now launch and proceed quite happily thinking it is running in XP SP2.
10. After the install is done You can close the "Create New Application Fix" wizard and the Compatibility Administrator tool without saving changes.
Afterwards, GVI and GVI-based products ran just as well on SP3 as they did on SP2 (for me, anyway!).
