Repartition C Drive Windows Server 2003
Repartition C Drive to solve low disk space problem in Windows Server 2003 is a common solution on system partition running out of space issue. When you backed up C Drive and resize system partition using command line before booting into Windows, you just ignored a simple way to resize C Drive.
Windows Server Disk management was not able to extend volume or shrink volume, Windows will not update this default tool neither, because in the year of 2015, Microsoft had ended support on Windows Server 2003, but that's not the disk managing tool I'm gonna use to solve low disk space problem, I'm mention this because I have better solution.
Partition Software Macrorit Disk Partition Expert Server can fix C Drive running out of space problem without repartitioning system drive; it manages free space on server disk without data loss. That's to say, you have the right to control free space, you can move them from larger partition to smaller partition, and you'll touch no files or data on the disk.
Click the above link to see step by step guide with screenshots in Windows Server 2003, this guide is also applies to Windows Server 2008 (R2) and Window Server 2012 (R2) when extend volume greyed out in Windows Disk management. In fact, Windows Disk management always fails to resize partition in many ways: New Simple Volume greyed out (New Partition Greyed out in Server 2003), Delete Volume Greyed out, and without surprise, Shrink Volume Greyed out. Here is the list of disk management functions greyed out and how to fix.
Besides, there's another way of resize C Drive in Windows Server 2003, using disk management alternative software, another partition tool from Macrorit, its Partition Extender, which extends system partition with one click. The fastest way to extend C Drive partition software, Partition Extender Server Edition, still extend C Drive without data lost or pre-step on resizing D Drive, you can directly click on C Drive, and Extend it. Here's the complete page of Partition Extender to extend C Drive in Server 2003, check it and download to have a try.