I formatted my system as it was very slow today.
If you are stilll running Lion, use Lion Recovery to reinstall the Mac OS X.
If you downgraded to Snow Leopard, you can reinstall Lion at no extra charge using the same Apple ID.
Launch the App Store.
How to re download apps from the Mac App Store:
Open the App Store. From the menu bar click Store > Sign In
Click Purchases from the top of the App Store window.
Select which apps you want to re download. Then right or control click where you see Installed then click Install.
Make sure and use the same Apple ID used for the original purchase.
edited by: cs