Arun, there might be one of a couple of problems which would cause the command to fail.
To begin with, do you have access to the root account and its password?
If the answer to this is "yes", then you might want to change file ownership with the "chown" command. For an arbitrary file called "junk.txt", the sequence might run like this:
root@home/steve$$ ls -all
-rwxr-xr-x steve users 2048 12-07-07 junk.txt
root@home/steve$$ chown arun junk.txt
root@home/steve$$ ls -all
-rwxr-xr-x arun users 2048 12-07-07 junk.txt
As "arun", you may not be able to delete my file named "junk.txt", unless you are logged in as the file owner or as root, even if the file is in a directory where you have full read/write/execute permissions.
Does this help?