It was after updating pc hardware, this is the new one:
Dual Core G620 2.6Ghz 3 MB LGA 1155
4GB 1333MHz DDR3
operating system: windows xp sp3.
No previous antivirus installed,just after a clean windows installation and drivers installed, , when I boot the computer the system freezes on windows logo, so no desktop loaded.
(Optional) If you want, save your avast! settings for later restoration (Settings > Maintenance > Back up).
Uninstall avast from Control Panel (if possible). If, for any reason, you can’t run it, try booting in Safe Mode and doing it from there. Anyway, boot after that.
Run the avast! Uninstall Utility saved on 1 in Safe Mode. Boot after you’ve run it. Repeat this for any major avast! version you have ever installed/upgraded in your computer.
Install avast! using the setup saved on 2. Boot.
Register your free copy or add the license key for Pro. Or even upgrade your key from old versions.
Check and post the results. If, for any reason, you did not solve, try doing the step 3 in Safe Mode anyway.
Yes, I posted on spanish forum, I did here too for having a wider range of suggestions. Spanish advisors are doing well. I take your suggestion and I will post here the solution. Thank you very much for your fast and kind reply.
You can follow ebozzz advice and repair Avast!. I do not know why you have to do it in safe mode with networking though. BTW if you already installed Avast! and ran aswclear a few times and still Windows hangs at boot I do not think a repair will do better.
You still got to answer a few questions in the Spanish forum.