Here's what worked for me when moving from an old PC running VirtualBox 1.5, to a new PC running VirtualBox 1.6.0 (both PCs running Ubuntu as host).
1. No need for disk cloning, just simple copy of old Win2k guest disk from VirtualBox 1.5 into the filesystem of the new PC (show hidden files and find folder .VirtualBox/VDI)
2. Start VirtualBox 1.6, create a new machine and link it to the copied disk you just installed.
3. Manually edit the machine before running it - look in General> Advanced> IDE Controller Type. Change type to PIIX3. (This should prevent bluescreen crashes on startup).
My Win2k guest from version 1.5 on my old PC is now working on the new PC in version 1.6 without any need to reinstall the whole guest OS on a new guest disk. I've read elsewhere on this forum that the same procedure probably works for a WinXP guest.
There can be some issues with screen resolution/ size. See thread at:
http://forums.virtualbox.org/viewtopic.php?t=6933