need help please dell latitude d820

Ok I am  kind of new to all this so please have a heart and  refrain from making me look stupid cause i might make it easy  by  being a easy  target . I have a dell laptop latitude d820 and my goodness it was / is slow . I mean so  slow  mail might be more practical using lets say the pony express so i do mean slow.  Then if anyone could be kind and  can help me get it to have better performance i would really be grateful. That is not the main problem,  due to it being so slow,  i searched ,  read  and saw its a good idea to reformat the entire thing and that  i did. I followed direction per windows tutorials and made a back up disk to reboot from. I then formatted the thing clearing it back to its virginity state and proceeded to boot from back up disk. I was informed i needed to authenticate my version of windows 7 so i used the key located on the bottom of my dell. It acted like it was going through and by that i mean it was taking forever and then eventually came back saying i could not use that key for my version of windows 7 pro. I dont have the money to just go buy a new one and it wont let me authenticate this version . Please someone,  anyone help me out here . To top it all off its even slower so it seems . Any education or enlightenment will be very welcomed and well received . Thanks and hope to hear all the things i need to bring this dell back to its working state.