Dell Inspiron 15 Intel help?

Quite a while ago, I had to get the hard drive of my Inspiron laptop replaced (manually, because I did not have the money to have it done for me). The hard drive replacement was supposedly the same size but when I tried to use the system restore disk, it says the hard drive was too small. So I downloaded Windows Vista instead, but realized that may have been the wrong choice because now all the laptop does it startup but I can't connect to any networks, ect. Can someone please help me restore my laptop?