how i can do unallocated partitioning of my hard disk

Would advise against using Wubi -- not intended for long term use.

If you really want to dual-boot using Win7 and Ubuntu, then continue reading ...

Confirm that you don't already have the 4 Primary partition limit. To do this, open the Win7 Disk Management utility and count the number of "drives" (that's what Windows calls partitions). If you already have 4, then you're going to be faced with a LOT of work because you will have to remove one before you can create another for Ubuntu.
Also, check the "drive" types and confirm they are NOT Dynamic Disks. Ubuntu can't be installed when Dynamic Disks are being used.
If you decide to go ahead with dual-boot, then do the following:
1) Use only the Win7 Disk Management utility to shrink your Win7 OS partition to make room for Ubuntu. Do NOT use the Ubuntu installer to do this. If you already have 4 partitions in Win7, do NOT allow the partitioner to create another partition. This will convert your partitions into Dynamic Disks -- effectively preventing the installation of Ubuntu and causing you further problems.
2) Once you have Win7 shrunk, use the Win7 Backup feature to create and burn a Win7 Repair CD. This will come in handy later, should you need to repair your Win7 boot loader from the dual boot setup.