how to install windows 7 on my laptop on which ubuntu waws already installed without deleating ubuntu ?

First, BACK UP YOUR DATA.

Next, boot from an Ubuntu live CD. Open GParted, shrink the Ubuntu partition to make some extra unallocated space on the drive, and create a new NTFS partition in the empty space. When you install Windows, be sure to specify to install on that partition (do not just let it install wherever/however it chooses).
AFAIK this should work; but I haven't installed Windows in ages.

Just to add to the reply above, windows will overwrite the master boot record code and you won't be able to boot Ubuntu so you will then either need to use the Ubuntu Live CD to reinstall Grub or modify the windows bootloader entry.

cortman
That was good. Backup
Then boot to a live cd also good. Shrink and install correct.
yancek is correct that you will have to fix the boot loader either windows or grub or other. Windows won't fix it for you.

Kind of other points too. You kind of need to consider maybe the position of the installation. Linux may have more than one partition, I'd guess you have at least two to choose from or more. Then you need to select which way to move them or shrink them. Some computers won't let you put windows too far to the end of the disk or one some types of partitions.

Other ideas are to run a virtual machine under linux.

Or install windows 7 then install linux. That is the more typical way. Then it is sort of automatic if you are lucky.

Good points; and glad for the mention of the MBR- you will need to reinstall grub on the MBR of the HDD (usually /dev/sda when booting from LiveCD I believe).

another good and easy-to-use tool to reinstall grub is Rescatux...