Do you still have access to the original XP PC? If so I would run through the process again, but this time run
MergeIDE before imaging the drive. The need for this is discussed
here, but you can ignore the antiquated imaging options suggested in that article.
Otherwise I have no problem with anything you did: Disk2VHD is designed as a disk conversion tool, so I'm pretty sure that selecting the C:\ partition gives you an image of the whole drive, so that should be OK - especially if it didn't offer any other way of specifying the drive. The main problem will be that VirtualBox almost certainly doesn't support your physical SATA controller, so having run MergeIDE it should now be possible to change the VM settings to attach the VDI drive to an IDE controller. This can be converted later to a VirtualBox SATA controller if this matters to you.
I have to disagree with Bill on one thing. While you
can use VHD in VirtualBox, it is definitely not recommended. You already have a process that results in a VDI, so I'd stick with that process.
Finally, if you have doubts that your VDI contains a valid partition map (i.e. the result of imaging a partition instead of a drive), then you can view the VDI using
CloneVDI. It can show you the partition map on the virtual drive.