Unfortunately the suggested fix did not work. The guest system still crashes every time I open an Excel file from the host shared folder, edit anything and save. I have obtained the hotfix from Microsoft, installed it but it did not solve the problem. Also, I have re-installed SP3 and all subsequent updates, disabled the webclient service - same problem. The "error reporting" utility from Microsoft suggested a buggy BIOS, because - quote message: "During the crash analysis, we noticed the basic input/output (BIOS) system version on this computer does not match the specifications for the central processing unit (CPU)". This might actually be a reasonable explanation, as my configuration is far from standard: XP SP3, running in VirtualBox 2.0.2 in Ubuntu 8.10 (2.6.27) on a MacBook Pro. Still, everything was running perfectly when I was using XP SP3 (absolutely nothing modified in the guest, I revert to a snapshot after doing whatever tasks are needed), VirtualBox 1.6.6 and Ubuntu 8.04 (2.6.24), same MacBook.