It is very common for server hard drive running out of space. No need to worry. Many people encountered the same situation.
As to the RAID 5 Array, is it raid card or the Windows built-in function RAID 5? It is easier if you raid is constructed by raid card. You can add the new hard drive to RAID 5 and then you need a partition software to add the free space to the drive which is running out of space. Then this article can help you undstand it better Resize c drive on RAID 5 without affecting data with step by step instructions
For Server 2003, volume manager is not recommended since it only supports Windows Server NT, 2000.
For a server partition software, here is here two tutorials with different software: Live test to extend boot partition on Windows Server 2003 with Paragon Partition Manager Server.
Safe and easy solution to extend server c drive.