For my surprise, when I opened VS 2005 Express and attempted to create a new project based on that kit, I found the kit wasn't shown in the IDE's projects browser.
The template was installed under this folder:
... but still the IDE didn't notice it at all.
At first I thought it had something to do with the fact that I'm using a non-english version of the IDE and a non-english version of Vista 32-Bit (something similar to what it's commented here: see section 1.2.2), but I cannot set the "International Settings" to "English" since, in my case, everything is in Spanish.
So, what to do? After locating the "SpaceWar" Stater Kit template file, I found that both, PC and 360 versions, where installed under a folder named "XNA Game Studio 2.0".
So, I created a new folder under ".../ProjectsTemplates/Visual C#", then renamed it to "XNA Game Studio 2.0", finally moved the "Net Rumble" template file to that folder, and presto!Cheers!