Difficult in which way?
For my desktop, it was originally a ME upgrade but eventually redid it to a
clean XP install. No particular reason. Have always done an upgrade first
to "test the water" and then gone on ahead with a clean install. During the
process, found two devices and a small handful of programs that the upgrade
advisor had missed and that needed updating to fix.
As far as using XP, there is a bit of a learning curve when coming from
Win9x or ME. However, there are enough similarities between the versions
that it is not too difficult to learn your way around the new and extra
features in XP.