Below is the history of what I did ,
1. I had Windows XP on my desktop.
2. Size of the hard drive is 40 gb
3. I formatted the hard drive completely.
4. Then I Installed RedHat Linux 7.1, by selecting automatically remove the data and format option.
5. After few days I again re-installed the RH linux 7.1, like 4 or 5 times.
6. Now I found that there is only 26 GB of space available on my hard drive instead of 40 gb.
7. I did check all the swap space allocated by linux during install, its not even more than 54 Mb space.
Can any one help me in undestanding what happened and also solution for this.