why do u want to host a mailserver on ur pc at home?
why dont u just buy some webspace connect the domain to it and their u can easily manage users, mailboxes and stuff.
plus u save money (energy), u got a much higher uptime garantee and its much more easy to configure if u do even consider to upgrade to windows 2003 (what the heck?) and what do u want with a fucking microsoft exchangeserver, that software is for companys and it doesnt really fit for ur needs.