How can I install windows 7 along with my existing Ubuntu 10.04 OS?
If you first install windows and then try installing ubuntu, you get both sistems working just fine. But if you try to install windows after installing ubuntu, windows installer will format your hard drive so you will loose your ubuntu installation.
There is a way to do it, anyhow, but is not easy and you will need a different physical hard disk. If your computer does not support more than one hard disk (most laptops don't) you won't get it working.
I've done it a long time ago, so I can't remember every step, but it should be something like this:
And that's it. If everything went ok, GRUB should recognize the new Windows installation, so you can choose it when you turn on your computer.
I hope that helps.