Had a problem with an XP (SP2) computer and Avast Internet Security. Upon installing the product (and its subsequent reboot), the computer freezes right after startup. The mouse still moves, but keyboard does not respond, no applications get started.

After a search on the Avast forum, I came across this post, claiming there's a problem with certain applications and Avast on XP:

Quote

This is caused by a bug in Windows XP function ImageEnumerateCertificates( ) which is being called by firewall service when a process tries to access the net. This function (unpatched in all version of XP) hang  ...