Upgrading to windows 7 question?

so i have windows vista right now and it stopped being able to update. that's when i found out it cant authenticate my license. i have a valid license but i let someone borrow the disk with the code on it and never got it back so i cant retype it or reinstall it. so the real question is can i buy the windows 7 upgrade if it considers the license not to be valid or will i have to buy the full windows 7 system? i can provide more info if needed