Here follows what I did, and has worked so far. I still have to see what happens after 30 days, but here goes the hint if someone wants to try. Instructions partly based in this video:
http://www.youtube.com/watch?v=r1TXPH0tZEkand partly in trial-error

http://images.thetechguide.com/forum/public/style_emoticons/<#EMO_DIR#>/rolleyes.gif\' class=\'bbc_emoticon\' alt=\':rolleyes:\' /> .
1.Install a Torrent client
2.Download last release of AntiWPA: isohunt.com/torrent_details/131409413/XP+SP3+%5BAntiWPA%5D+Activator?tab=summary
3.Download Windows Xp Genuine Forever 100%: isohunt.com/torrent_details/60720773/Windows+Xp+Genuine+Forever+100%25?tab=summary
4.Download WinXP SP3 network install version* if needed
5.Log in as Administrator, if not yet
6.Install SP3 before changing anything** (check
http://support.microsoft.com/kb/950717/en-us before)
7Website removed for spammingn AntiWPA
(Now WinXP is activated, but your license key may be still blocked)
8.From "Genuine" downloaded files, copy all contents from "System32" to your actual System32 folder (usually C:\Windows\System32\)
9.Still from "Genuine" files, in folder System32, run 1.reg - Say "Yes" to the registry change
(Now you own a "genuine" install and can flawlessly run Windows Update)
*Installing SP3 from Winblows Update didn't work for me. Maybe a different key would do, but I didn't try.
**Installing SP3 after patches gave me "Internal error" which, according to the logs, is due to licensing. Maybe a different key would do, but I didn't try.
Note 1
In case you already patched your system and want to install SP3, you can follow advice from M$ itself:
http://support.microsoft.com/kb/949384/en-usOpen C:\Windows\svcpack.log in Notepad or similar, Search "CheckVLKForBlock failed" and read the lines right above. Follow instructions from link above. In my case, it was the second case, and the solution worked.
Note 2
According to Avira's AntiVir, there's nothing wrong in the patches. But check for yourself.
Note 3
I've seen FittMunken's guide back in this thread (
http://www.thetechguide.com/forum/index.php?showtopic=10100&st=183) but it didn't seem necessary (and some reported as nonfuncional). I don't know if it makes a difference, and didn't try. Maybe would solve the SP3 issue above, maybe not. I'll leave that to the experts.
That's it! Let's pray for the thing to work (specially after 30 days). Well, that's what we always do about Windoze, isn't it

http://images.thetechguide.com/forum/public/style_emoticons/<#EMO_DIR#>/tongue.gif\' class=\'bbc_emoticon\' alt=\'

\' /> ?
Best regards,
Me