I have installed ubuntu 10.04 on my laptop manually. i.e. with booting the live CD and by allocating the partitions in a different partition.
Now I want to remove it and use that space, so I want to install it through windows so that I can use that extra space. What should I do now?

You can't install Linux "through Windows". Windows thinks it is the only operating system in the world. You have to install Windows first on a primary partition, then install Linux subsequently on a different unused partition on the same HDD.

Doesn't Ubuntu have a windows installer, a windows program that will install Ubuntu on your system? But I believe it does the same thing as a install cd. Someone correct me if I'm wrong.

Yes, that would be "Wubi" - though I haven't a clue into which versions of Windows it will install, nor with which versions of Ubuntu it works, if not all of them.

looking at http://wubi-installer.org/, it seems wubi installs itself as a windows app without changing any partitions. I think this is what you're looking for Yogesh.

As for what to do with the extra space, what do you want to do with it? You could install another OS, create another windows drive, or use gparted and expand your existing partitions.

You just got me wrong! I have Windows installed already on my system. In a separate partition, I have installed Ubuntu. Now I don't want this Ubuntu. I use both Windows and Ubuntu. But the space which I have given to the Home folder is more and I can see that space in Ubuntu only. This partition is not visible in Windows(ext3).
Therefore, to utilize and rearrange the space, I want to install through Windows (Wubi as said above). This is what I want.
You know, the problem lies in the fact that the Windows space is visible in Linux but the Linux space is not visible in Windows.

You just got me wrong! I have Windows installed already on my system. In a separate partition, I have installed Ubuntu. Now I don't want this Ubuntu. I use both Windows and Ubuntu. But the space which I have given to the Home folder is more and I can see that space in Ubuntu only. This partition is not visible in Windows(ext3).
Therefore, to utilize and rearrange the space, I want to install through Windows (Wubi as said above). This is what I want.
You know, the problem lies in the fact that the Windows space is visible in Linux but the Linux space is not visible in Windows.

...m

As the guy above says, it might be worth removing your Linux partition and just running what is called an "Ubuntu appliance" through VMware for those odd occasions when you need Ubuntu. Assuming you have enough RAM and processor power for what you want to do, of course.

I would prefer to use linux by installing through Wubi installer... but I have a doubt. I think that directly formatting the linux partition will erase grub.. then how will it find windows when I start it again? Is it that grub also will get erased and hence Windows will be automatically booted?

The wubi installer will put Ubuntu inside windows, so if you install it, your current Ubuntu partion(s) wont be affected. The difficult part will be uninstalling the Ubuntu partitions and, as you mentioned, dealing with the bootloader. Have you decided what you want to do with the Ubuntu partition? If we know that, we can help you recover that space.

To talk about this point for a moment, you could create a partition to be shared between the two OS's. If you create a partition and format as either NTFS or FAT32, both OS's can mount it.

Yeah, I have decided what to do to that partition. I want to merge that into windows to free the space as I said before. I think formatting the partition can be done using Ubuntu live CD or windows live CD. What's difficult is dealing with the bootloader. I just don't know what to do with that.

I don't know of a windows tool that can resize partitions, so I think the best way to combine your partitions is booting a Linux LiveCD and using a tool like gparted to delete the old partition and extend the existing windows partition.

But if you want to keep your partitions as they are and just reformat so that windows can detect and use them, then you should just be able to do that within windows, but not being a windows user, I'm not too sure the best way to do so.

You can restore the MBR using a windows disk in rescue mode, but I'm not sure what the command is, something like fixmbr or something. But that's easy enough to google.