Windows 7: installing windows 7 on new hardrive

Please can someone help me!
I am trying to install windows 7 home upgrade onto a new hardrive i have bought for my laptop,but it says the product key is invalid.I purchased the genuine windows 7 software from asda/walmart,and orginally installed it on my laptop which was running vista,so i just upgraded using the windows 7 upgrade software which worked fine.Then i had so many viruses etc,i decicded to buy a new hardrive and then load the same windows 7 home upgrade software by doing a clean install,but when it came to activate the software it gave me a code and said error product key not reconised.

any help

Try this: Clean Install with a Upgrade Windows 7 Version