Step1:Open Computer management by clicking START > Control Panel > System and Maintenance > Administrative Tools > Create and format hard disk Partition. Or, Go to Run command and write "diskmgmt.msc" hit enter.
Step2:Right-click (C:) Drive on your hard disk and then click Shrink Volume.
Step3:On the New window Type the size of the volume you want to create in MB (40GB =40000 MB) on the "blank space" after "Enter the amount of space to shrink in MB:"
Step4: Then click Shrink and follow instruction.
[Note: This instruction was based on Windows vista home premium]