My problem is I have ubuntu 11.04 installed on my system. I have no other os installed. I have tried to install windows xp using bootable cd of XP. But it doesn't work. Blank screen appears. How can I remove ubuntu then? I have tried many ways using gparted and all but no result.
3 possible methods for removing an operating system from your hard disc (so it does not even have to be Ubuntu):
You can remove the current OS from your hard disc by using the Live CD you used to create the installation. It will have gParted installed and loaded into memory and all you need to do it delete the partitions gparted shows you. Mind you: you can only delete unmounted partitions. And not all operating systems will have a live CD.
You can also use the gParted Live cd that you can find here. You need to burn this though. So if you can not access your current OS you might need a 2nd computer to burn the disc.
You can also use another OS installation media to format your disc. All operating systems installation disc will have some sort of partition manager. Windows starts with it when you boot from the installation CD. Linux flavors tend to ask for some information 1st (like language settings etc) before you see the partition manager.
If you decide to stick to gParted then this link explains how to do that. Copy/paste from that link:
Deleting a Partition
To delete a partition:
Select an unmounted partition. See the section called “Selecting a Partition”.
Choose: Partition → Delete. The application displays the delete partition operation in the Pending Operations pane.
Do not forget to read the "caution" part underneath these pointers.