The normal method (http://support.microsoft.com/kb/325590/en-US) using diskpart doesn’t work on the C drive. Supposedly if you can put in the server 2003 boot disc you can make it work, but I did not have that option.
Dell has a tool called ExtPart that may be able to help you, especially if you are working remotely. You can download it at the following link: http://www.dell.com/support/drivers/us/en/19/driverdetails?driverid=R64398 (now located in Resources / Hard Drive & Partition Tools)
Make a backup of everything you can first, just in case.
This may also work on XP or 2000 servers but they must be 32 bit.