I believe you can also take a snapshot in Virtualbox immediately after importing from the OVA. Reverting to that snapshot should effectively rearm the VM without having to keep the OVA around either. Of course, that won't work for the hard-expiration in the XP VM.

Rey, it looks like there are actually 5 rearms available, but only a 10-day trial judging from "slmgr /dlv": https://dl.dropbox.com/u/463624/Screen%20Shot%202013-02-04%20at%203.22.13%20PM.png
I don't know much about Windows licensing under the hood - is that correct?