If you have Windows 7 or Vista and your hard drive has been partitioned into different drives, then you can repartition the C: drive to make it larger.
For example, your hard drive is 640GB. The C: drive has been partitioned to be 60GB, the rest of the 580GB is used to create a logical D: drive. Logical means the D: drive is not a separate physical hard drive. Win 7 and Vista has a built in function that allows you to increase the size of the C: drive by taking storage space away from the D: drive and adding it to the C: drive. You can increase the C: drive to 80GB and shrink the D: drive to 560GB. Total capacity remains 640GB.