WTF is wrong with this?

today in the morning i bought os x lion from the mac app store. when i was downloading it i think something went wrong, because i looked how the pregress was going and the download was very slow, the next minute it appeared it was complete. when i tried to install it, i couldn't because this message would appear "The software could not be verified. It may have been corrupted or tampered with during downloading." what do i do? i tried to press download again in the app store but i couldn't because i had already downloaded it....... ughhhhh help pleaseee