There is no hard requirement that there be such a drive at that letter. Long discussion but why not tell a little more?
While reading http://social.technet.microsoft.com/Forums/en-US/w7itprogeneral/thread/7fe04a5c-2ba8-4490-9f80-ec15a2e2483c is interesting it appears to happen to those that upgrade. But your post details are very slim.
I was going to get around to defragging my C: drive today, (which I don't actually remember if I've ever done in Windows 7) and when I opened the Disk Defragmenter, the C: drive doesn't show up, and as a result I can't defrag it. Any ideas as to why this is? I have an HP Probook 4510s, if that helps.