xp rebooting loop

since I installed version 6, my pc is running in loop on startup. After formatting the hard disk (not start in safe mode or whatever) and reinstall XP it restarts properly and works properly until the fateful moment where I reinstal avast version 6 and the worries recomm unannounced stop and restart loop
I tested with 2 hard drive and the problem is recurrent

could you m’appoter a solution to the problem please thank you
ÉcouterLire phonétiquement

Which other security program are running?
Other antivirus? firewall?

Which avast…?
Which version…?