ok dude, the thing is that only 4gb is assigned as partitioned space, the rest of the space is unpartitioned raw space.. to fix it you either need to go to control panel>administrative tools> computer management> storage then you need to assign that space as a partition
OR
boot up using a windows vista or xp disk then follow the steps and you will come to a place where it gives you the information about the hard disk and it will show like Partition C: 4gb Unpartitioned space:156gb . then select the option to create a new partition then make another partition out of that space. if you have windows vists disk you can increase the size of ur current partition "C:" without having to create another one or losing ur data on "C:"
hope that helps