Hi, welcome to the forums.

I use Quicken 98 in compatibility mode (win98) with XP Pro SP1a and no problems with avast!

This could be something to do with the QW3 being a 16 bit program there have been a number of posts about running old programs. This could possibly be a similar problem.

A search of these forums returned some info that may help.
http://forum.avast.com/index.php?board=2;action=display;threadid=7651;
http://forum.avast.com/index.php?board=2;action=display;threadid=6991;start=0

Applying Vlk’s reg fix in the second thread above will probably resolve it, do however read both threads to get the full background.

HTH David