I moved from AVG to avast and like many others it found virus that had lived hapilly alongside AVG.
Many have reported the same sort of thing (finding existing virus infection) after the first boot scan in winXP after installation of avast. Norton's NAV is a pig to get rid of and if avast finds another AntiVirus program installed it may refuse to install fully.
I don't believe this is avast being restrictive, it is simply trying to ensure that the two programs don't conflict, posibly leaving you more vulnerable.
Some AVs can be on the same system, but are not active, you can do a search on these forums for further information.
I would recommend that to get you up and running safely that you uninstall other AVs prior to installing avast.