i have installed windows and ubuntu in my hard disk, what i am wondering is how can i delete ubuntu and install another linux distro without screwing the GRUB, because before when i had deleted the ubuntu partition grub got corrupted and i couldn`t boot in to windows.
after i delete ubuntu i just install the other distro in the free partition?.

Look Grub needs a bed to sleep in.

Ubuntu is currently providing the bed for Grub.

If you nuke Ubuntu you take away Grub's chance of return.

If you install another distro it will provide the facilities that Grub needs but it may be another Grub.

Realistically you should learn how to repair Grub and a Ubuntu CD is all the tool you need to bring Grub into submission.

I wouldn't loose sleep if Grub is gone because you can recall the bugger 100 times. You do need to have a place for it to sleep in first.

If it helps you can put Grub in a data only primary partition in fat16 or fat32 and Grub will serve you when there is no Linux in sign. Also you can ask Windows to boot Grub too.

Here is an example of one Grub booting two Vista without a Linux installed.