If you have OS X Lion installed, you should be able to access the Recovery HD which has the ability to download a fresh copy of OS X Lion from Apple Servers. the only thing you have to do is to start up your Mac and when you hear the "Bong" sound hit the "Option" key to show the bootable disks in your Mac. You should see two bootable disks one should say Macintosh HD and the other should say Recovery HD, if that's the case, with you arrow keys select the disk and then hit the "return" key. An Apple logo should appear and then you should see a screen like this:
Credit: Apple, Inc.
Go to disk utility and when it opens select the HDD (not the Macintosh HD partition). Then look for the Erase tab which should look like this:
Select the partition format as "Mac OS X Extended (Journaled)" and then click Erase. You should see a warning message saying that the action will result in the deletion of all partitions and data in the drive, click OK.
Warning: This will delete all your data, document, user settings, etc., that is located in your hard-drive. Make sure to always keep a back-up of all essential user data, applications, documents, and settings that you wish to preserve. Apple, Inc. nor this author is responsible for the consequences of not backing up your data. Do it at your own risk!
Close Disk Utility...
In the main menu, select the option of Reinstall Mac OS X Lion, follow the instructions of the assistant and agree to the EULA; select your installation drive and Mac OS X Lion should start downloading into your disk (the time this process takes will be determined by the speed of your Internet connection). Your computer will reboot after downloading and install OS X Lion. When installation is successfully finished, your computer will reboot and lead you to the OS X Lion set up assistant.
Well that's very much it. For more information about the Recovery HD visit:
Hope this helps!