What problems have you faced when registering and activating Windows XP under VirtualBox or similar?
The standard XP activation rules apply (Ars Technica had an article on XP activation rules back-in-the-day). The best thing to do is to set up your virtual machine as required (RAM, main HDD) before activation, then create a snapshot of it; for any subsequent changes (eg, extra disks), revert to that snapshot.
If VirtualBox supports any differencing mechanisms, utilise those, too.
No full DirectX support.