Just a little note about my last VirtualBox upgrade to 3.1.4. After the upgrade and upgading the Guest Additions, the guest (Windows XP) became painfully slow.
The reason was very very bad disc performance. Eventually I removed all shared folders and that helped. I can’t remember if a reboot was necessary or not, but now things are almost back to normal (network is still slow and external stuff that is not GET in a browser will just timeout!).

If anyone has more about how to fix this I’d really appreciate to hear that…