You can't uninstall the OS. And you can't just transfer the hard disk.
What you should do: a clean install of Windows 7 from the disk. That will try to activate. It will be refused, because it's already activated on that other machine. So you will have to call Microsofts free activation-issues phone number, explain the situation and they will either remove the current activation from their database or give you a new license key that won't be refused.
The alternative is buying a new license, but that'snot what you bought the retail version for. If that was an upgrade version, however, it might be necessary, because it's doubtful if an upgrade version can be reused in a new machine.
I installed a retail version of W 7 on a Fujitsu which broke 6 months ago and cannot be restarted. Now I cannot activate the installation on my replacement computer and the support is less than useful, always referring back to an automated system. I transferred the HD from the old computer to the new but cannot see a way to uninstall the windows 7 from that disc. Trying help by telephone was impossible because of excruciating bad connection. Any suggestions?