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OK... Here are the news: I have installed ActiveState 
version without problems. I had to: how the hell could I 
compute a MD5 digest of the Python installer with a Python 
script, if I can't have Python installed, hm?

Anyway the digest proved to be perfectly correct. After 
all, I tried downloading the file for 2 different sources, 
and this was not a copy of the file I used on Win2k (see 
request ID 416824). But computing this digest allowed me to 
find the origin of the problem. I realized that I had saved 
the installer on a remote drive (windows shared directory 
on our file server), when I had to type "V:Python-21.exe" :)

My guess is that the SYSTEM account must have the right to 
read the directory containing the installer, and it was not 
the case. Once copied locally, it seems to work (I didn't 
go through the entire process, though, as I already had 
Python installed before).

I don't know if using the msi format is free or not. 
Another way for M$ to make money with a new mandatory 
installation format? If it's free, then it's the obvious 
choice for a future version of the windows installer. I'll 
try to gather some informations about it.

And "as to believing you", I must apologize to you: this 
sentence was a dumb litteral transcription of a french 
idiom which doesn't have exactly the sense it seems to 
have. This was just a way to tell readers that this 
information did came from you, so having a look at what you 
said may be of interest. No harm intended, really.