avast does scan P2P programs, just not all of them with win98, check the list in the help file without an * they will be ok with win98.

Extract from Help file.

Note: Protection of the programs marked with an asterisk (*) is available on Windows NT, 2000, XP, and 2003 only.

As far as I am aware, this restriction is more to do with those P2P programs using the XP functionality, rather than avast not scanning them.