By resizing the virtual disk you have only done part of the job. Now you need to get the guest OS to recognize the added space. To confirm this, right click on My Computer and select Manage, then click on the Disk Management item in the tree on the left. You should see your C: drive as the new size, but the active partition is the same as it was before you increased the disk size.
Take a look at this document, it gives you step by step instructions on how to do the entire job. If I understand where you are at, you need to increase the size of the Primary Windows Partition. I personally have done this with Gparted a few times without problems. Read through the document and follow the instructions and you should be all set.
