The system drive C is the default location of the hidden system file pagefile.sys. You don't say what version of Windows you have with this problem. The default size of the pagefile.sys varies according to Windows version if you let Windows manage it. In some cases, it can be equal to the amount of RAM. For example, if you assign 8GiB RAM to the VM, it will create pagefile.sys of 8GiB in the system drive.
You can manually set the pagefile.sys to a fixed size so that it won't get created/destroyed everytime the Windows VM is booted up/shut down.