Microsoft Knowledge Base Article 842715 contains the following summary:
This article describes the symptoms that may occur when one or more of the hidden administrative shares are missing on your computer. The article also provides information about how to resolve this problem.
If you have already determined that your computer is missing one or more of the hidden administrative shares, see the "Cause" and "Resolution" sections. Realize that missing administrative shares typically indicate that the computer in question has been compromised by malicious software. We recommend that users format and reinstall Windows on compromised servers.