Hi! I'm using a computer at work which someone just left behind. It originally came with Leopard (it's a white MacBook I guess from around 2009), and we installed Lion last week. It worked fine, but someone decided to change it back to Leopard. We used an original startup disk from another MacBook, and it said it couldn't be installed. I'm a little lost here with what exactly happened next, since I'm not the only one using this computer. Anyways, now I have a Lion DVD but it just won't work. It pops out an error message that says that the OS X 10.7 is not available. And it won't let me go past the utilities screen. I've verified it, tried to erase the partitions (which can't be done), turned it on holding down all sorts of keys, but nothing seems to work. The HD doesn't have any errors, and when I go to disk utilities it recognizes the HD, the DVD, and I even connected an external HD, but when I try to change the startup disk, I can't. They just won't show up, not even the Mac HD. I hope I could explain my problem since English is not my native language 
Oh, and taking it to the Apple store is not an option where I live, btw. I'd really appreciate it if someone could tell me what to do! Thank you!!!
Oh, and taking it to the Apple store is not an option where I live, btw. I'd really appreciate it if someone could tell me what to do! Thank you!!!