Allocating more space to the C: drive

I have a server running MS Server 2003 for Small Business. When it was originally setup,the boot partition (Local C:) was allocated for only 12GB where drive D: was allocated for 136GB. I am now very low on disk space on C: and I would like to add more disk space to C: by taking from D:. I run many applications off this server to support e-commerce. Any solutions or suggestions would help me out a lot. I have just over 100MB on C:, so I’m looking to do this ASAP.