In order to increate the C: partition in your Windows server VM, you need adjacent free disk space.
In case there'the virtual disk only contains the C: parition, the follow the steps provided by jsider. Note that a VM reboot may not even be required, if a rescan within the guest's Disk Management detects the increased virtual disk.
If there's anther partition on that virtual disk, you'll have to move that partition to get the required adjacent free disk space for increasing the C: partition. This can either be done using a 3rd party product like gParted, or you can use VMware's Converter which allows you to clone the current VM, and do the resizing in a single step (Volume based V2V conversion). Using the Converter has the advantage that the original VM remains untouched, i.e. you can always revert to the original VM in case something doesn't work as expected.
André