Reinstalling factory system

Hello
i have been using window vista home premium that came with my computer. my computer is a compaq presario. i have been having a problem with the window vista that came with the factory setting and i insatalled an old version not original cd just a copy and it is asking me to activate a product key which i dont know. is it possible to have back my previous factory system and how am i going to do it if possible. all the files in drive D have been deleted but in drive C there is window old program. please i need some help.