If you have a legal windows 7 key you need to download a legal windows 7 iso and reinstall then you will have an OEM windows 7.
You have a dell logo because you used a copy of dells install of windows 7. Then your question makes it sound like your installing this on a non dell system which is not the original system which is piracy which is not supported on this forum.
To reply to you and bignastyid I bought this copy of windows from amazon for 45 GBP. If it is a pirate copy I will be immediately reporting the seller to amazon.
Thanks for you help guys.