If that doesn't work, you can use diskpart: Be carefull, because you have to select the correct partitions to delete.
1
Open a command prompt on your computer by clicking the "Start" button, clicking on the "Run" button, typing "CMD" (without quotes) into the command prompt, and then pressing the "OK" button. If you run Windows Vista/7, type "CMD" into the Search Bar that appears when you click the "Start" button and then hit the "Enter" key.
2
Type "diskpart" (without quotes) and press "Enter." You are now in the DiskPart utility.
3
Type "list disk" (without quotes) and press "Enter" to see a list of hard disks that are on the computer. Type "select disk=x" (without quotes). Replace "x" with the number of the disk that you wish to select. Press "Enter."
4
Type "list partition" (without quotes) and press "Enter" to see a list of partitions on the selected hard drive. Type "select partition=x" (without quotes) and press "Enter." ("X" is the partition number you wish to select.)
5
Type "delete partition" (without quotes) and press "Enter" to delete the partition that is currently selected. Once you have deleted the partition that you want to delete, type "exit" (without quotes) and press "Enter" to exit the DiskPart utility.