Pitfalls of Windows XP Virtual Machines
Solution 1:
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.
Solution 2:
No full DirectX support.