In fact, after reading the board I can say that:

  1. AVG 6.0 (free) and 7.0 work with avast in Windows 98 and Millenium
  2. AVG 6.0 (free) does not work with avast in Windows XP/2k. In some XP systems, AVG 7.0 worked with avast… I could not configurate both of them to work :-\