To access the disk partition manager in windows you will have to either right click My Computer and select Manage from the menu; which will bring up the Computer management console, where you will find disk management.*
The other way is to click the Start Menu and select Run. Type in the run box CMD and hit OK. When the command prompt come up type in diskpart to load the text based disk partition manager once in the program you can type help to find out about all of the commands available.
If this is still not helping you should be able to download the program Partition Wizard which will give you a lot more options as to resizing your disk partitions.