Can an already XP activated computer deactive after a couple of months ?

About four months ago I installed XP Home on a PC, I activated it using a legit key. It was activated, I even used that command to check the actication (oobe/msoobe /a) and it said XP is already activated. I even installed that windows validation tool that usually installs with windows update, I ran that program and it said it was a legit copy. So, now four months later I turn on my computer and I get that windows at startup asking me to activate XP ??? Why is that ? Is it possible to lose an XP activation ? I have never heard of that ? Is it possible my IP address is blacklisted by microsoft or something because I have activated other computers using other keys though....Can anyone comment on this ? Thanks.