This morning I decided to upgrade my copy of Sun's VirtualBox on my Win7 (RC) machine. I had been putting it off for a while but decided it was a good time to do it.
After downloading and installing the new executable, I thought it wise to test my virtual machine was still in working order. It was not, but this fix is simple. I got an error telling me that the Guest Additions file could not be found.
The solution is as simple as pointing the mounted ISO file to the proper location. Click for the full text with screenshots and further explanation.