I was playing around with a VPC image for windows 2003 server. I created a 'blank' image and wanted to copy this, do I wouldn't have to go through the install each time I wanted to try out some new server stuff. I copied both the config file (.vmc) and the hard disk file (.vhd) for the vpc to another location and tried to re-start this new copy. Instead of the usual bootscreen I was confronted by a "NTLDR is missing. Press any key to restart..." message. I almost freaked and switched to using VMWare ;-)
The solution to this error is simple: instead of copying both files, just copy the hard disk file (.vhd) and create a new virtual pc configuration on top of this copied hard disk. The cause is probably some name or path settings in the vmc file that ar no longer in synch with its location. Creating a new vpc config is as easy as starting up the vpc console, clicking 'New' and following the wizard. (this even works after you get the error, there's nothing wrong with the hard disk file)
EDIT: turns out I was wrong. The VPC tried to boot from a USB stick I had left in one of the slots and didn't find a boot sector. Removing the USB stick removed the problem