When you look at the properties of the drive, what does it say the total capacity is? If it says the total capacity is approximately 40gigs, it's most likely that your partition is only 40gigs in size and that the rest of the drive is unpartitioned.

You can check this by right-clicking My Computer, selecting Manage, selecting Disk Management, and looking in the lower-right pane. Look to see if your disk is listed with two different partitions, with one saying 40 GB NTFS and the other one saying Unpartitioned Space.

If this is the case, you can create a new partition out of the unpartitioned space by right-clicking the unpartitioned space and selecting Format. This will create what appears to be a new drive on your system. You may wish to have all the space in one large partition, but you cannot do that without either purchasing partitioning software or using freeware software that is should only be used by advanced computer users.

In my opinion, it is better to have a secondary partition. Save all your must keep files in the second partition. This way you can format the partition that Windows is on and reinstall the operating system if needed, and you will still keep all your needed files.

Let us know if this addresses your issue.