VirtualBox Guest Additions won't resize Windows XP guest

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 yesterday when 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 from Sun (PUEL) and reinstalled the Guest Additions without it changing anything. Any suggestions?

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. Since I tried two versions of VB, I am tending to assume the problem is with Xorg, but it's a guess.

I tried creating a new xorg.conf using "X -configure" but starting X using this xorg.conf produces a black display. Otherwise, Xorg itself continues to work normally.

I searched LaunchPad and tried disabling the VT-x extensions as mentioned in bug 455112. This had no effect.

Thanks.

ProblemType: Bug
Architecture: i386
Date: Thu Jan 28 21:27:22 2010
DistroRelease: Ubuntu 9.10
MachineType: Hewlett-Packard HP Pavilion dv6000 (GA449UA#ABA)
NonfreeKernelModules: wl nvidia
Package: xorg 1:7.4+3ubuntu10
ProcCmdLine: root=UUID=c1a52e93-8108-4241-857b-ab9c8bd03b25 ro quiet splash
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-17.54-generic
RelatedPackageVersions:
 xserver-xorg 1:7.4+3ubuntu10
 libgl1-mesa-glx 7.6.0-1ubuntu4
 libdrm2 2.4.14-1ubuntu1
 xserver-xorg-video-intel 2:2.9.0-1ubuntu2.1
 xserver-xorg-video-ati 1:6.12.99+git20090929.7968e1fb-0ubuntu1
SourcePackage: xorg
Uname: Linux 2.6.31-17-generic i686
dmi.bios.date: 03/09/2009
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: F.42
dmi.board.name: 30B7
dmi.board.vendor: Quanta
dmi.board.version: 65.2C
dmi.chassis.type: 10
dmi.chassis.vendor: Quanta
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnHewlett-Packard:bvrF.42:bd03/09/2009:svnHewlett-Packard:pnHPPaviliondv6000(GA449UA#ABA):pvrRev1:rvnQuanta:rn30B7:rvr65.2C:cvnQuanta:ct10:cvrN/A:
dmi.product.name: HP Pavilion dv6000 (GA449UA#ABA)
dmi.product.version: Rev 1
dmi.sys.vendor: Hewlett-Packard
fglrx: Not loaded
system:
 distro: Ubuntu
 architecture: i686kernel: 2.6.31-17-generic