When I used WinXP Home it happened more times (on different machines) that it locked itself, because the activation time expired.
I downloaded some guides and patches, but most of them didn't work.

Finally I found a page that describes the bytes that should be altered in the winlogon.exe and it will think that the product is registered.

The WinLogon.exe crack worked in all cases.

Guide:
1. boot in safe mode
2. locate winlogon.exe
3. remove the one in the dllcache directory if you find it there
4. use a hex editor and change the appropriate bytes
5. reboot

To obtain the exact offsets and data for point 4 search on www, different versions may need different patch.
Be sure that you are using a good hex editor that just changes the bytes you are intended to change. Hiew (Hacker's view) is a good choice.

If you can enable the Internet connection in safe mode (I could), you can find/download everything (patch offset/data, hiew) that is needed to use your op.system.

Good luck for it, for me a full reinstallation of all the problems that I used would have taken a whole day, this patch took half an hour including the search on Internet. Not to mention the possible risk of losing valuable data when format.