I have for years been using VirtualBox under Linux (currently Ubuntu 9.10) to run Windows XP Pro as a guest OS. It worked fine until today, for some reason, the Windows guest stopped resizing to match the size of the VB window. Also, the maximum resolution permitted by Windows is now 800x600. I have upgraded to the latest stable VirtualBox and reinstalled the Guest Additions without changing anything. Any suggestions? I'd rather not revert to the last snapshot if possible because I've installed lots of software since, e.g. Adobe Flex Builder.
EDIT: I tried starting a CentOS virtual machine and while it works fine in text mode, it cannot display in X (black window), indicating that the problem is either in VirtualBox itself or in Xorg. Ubuntu did update Xorg recently.