If you are using Windows Vista or Windows 7 then you can create the partitions from the “Disk Management”. If 120GB is allocated to C drive and if you want to shrink that size to create a another drive then
1) Click on Start –> In search box type diskmgmt.msc and press “Enter”.
Disk Management window will be opened.
2) Now right-click on your OS partition ( C drive ) and click on “Shrink Volume”.
The Shrink Volume window will appear. Type the amount you want to shrink by in the ‘Enter the amount of disk space you want to shrink in MB’ label.
For example, to shrink the partition from 35 GB to 30 GB, type 5000 in the text box and click on “Shrink”.
3)Now right click on the unallocated partition or volume and click on New Simple Volume and follow the instructions.