How to change Username in Mac system?

I have an iMac which is running on Mac OS X Snow leopard. Recently the hard disk of the iMac crashed which lead me to change the damaged hard disk. The person who replaced the hard disk and installed the operating system on it did not keep any password and kept the default user name as User. Now I want to keep password for my system and also want to change the user name of the computer. So I want to know if it is possible to change the Username in Mac system and if it is possible then How to change the User name in Mac system? Please suggest some solutions

It is very easy to set the password in Mac operating system. If the system is already set with the password and if you want to reset the existing password, Insert the install disk of the operating system which you are using. After selecting the language to boot. Now from the Utilities menu, select the Password reset. Now enter the Administrator account user name and password. Now select the account for which the password has to be changed and then enter the current password and the new password. Once done, save the settings and the exit the installer. Now restart the computer, the new password will be set.

Instead of changing the username of the current account, you can create a new account with administrator rights. Creating a new account with the name you want will be more easy approach. To create the new account, go to system preferences and then select user accounts. Type the name which you want and also assign the password if needed. Remember to assign all the administrator rights to the account so that you are able to use all the features when you use this account to log in to the computer. Once the account is created, transfer the files which you require form the User account and then delete the account.

Follow these steps to change the username in Mac Operating system which is later than 10.4 :-
1) First you will have to enable the root user.
2) Once the root user is enabled, log in as root.
3)Now go to the /Users folder.
4)Highlight the Home folder and rename it with the name you want to change.
5)Now create a new account with the same name which you have given to the folder.
6) The system will prompt you to use the same folder for this account. Click OK.
7) Once this is done, exit the root user and then restart the computer.
Before following these steps take the backup of the data.

To change the User name in Mac OS X snow leopard follow the steps :-
a.Go to System Preferences and then open the Account pane.
b. Provide the necessary user name and password with admin rights.
c. Now from the list of the accounts, control click on the account whose name you want to change and select advanced options.
d. Now click on the (+) button under Aliases and type the name which you want to be the username
e. Click on OK and restart the computer.

Thank you all for replying to the question. By referring to the solutions provided by you I was able to set the password and also change the User name of my computer. I set the password using the Install disk method and then followed solution provided by Dammit which turned out to be really easy.