Could somebody tell me where I can find the following file to adapt Vision Command onto XP : Vision Command Updater.exe (from Robotics Invention System 2.0) ?
Thanks for your trouble.

To be honest, I don't think Vision Command supports XP.
Anyhow, I'd recommend you to address your question to MindStorms Support

http://mindstorms.lego.com/support/contact.asp

To install on XP, do the following:
1) stick the cd in. Do not install yet - close anything that automatically runs.

2) In windows explorer, view the contents of the cd.
(or use my computer, and 'explore' the cd)

3) right click on vision command's setup.exe. Select 'properties', then the Compability tab.
Now click on The compatibility mode checkbox, and in the drop-down box, select 'Windows 98'.

hit apply, then OK. Now double click on Setup. It will now install. Have fun!