VMWare has VCenter Converter for converting physical machines to VMs.
VirtualBox's vboxmanage only converts between disk types - it doesn't inject drivers that may be needed to boot the disk in the VM, which is why you're most likely getting the INACCESSIBLE_BOOT_DEVICE blue screen. XP is especially problematic in this regard, since it usually used hardware-specific disk controller drivers. Try following these instructions, they may let you make your XP bootable.
