After an upgrade of VirtualBox  to the latest 2.2 version (beforu 3.0 release), all the XP guests stopped with bluescreen during booting. The message was sthg. like check your harddisk etc… The host filesystem was OK, however I did not manage to check the integrity of the NTFS filesystem on the virtual disk. Few days ago the VirtualBox 3.0 came out. Downloaded, installed, the same error.

Problem found: the IDE Controller Type was changed from the old one PIIX3 to PIIX4. Changing back fixed the problem