Any virtual machine software (free or otherwise) such as VirtualBox will work.
Ideally, obtain a copy of the Windows install media and reinstall Windows cleanly to the virtual machine as the hardware will appear different to the current installation of Windows. Copies of the install media are usually available from your computer manufacturer or directly from Microsoft for a nominal fee. You could also borrow installation media from a trusted source such as a friend or colleague. You may also be able to purchase install media via eBay or similar although this is a less trusted source.
You can retrieve your current product key using a free utility such as one found at: http://pcsupport.about.com/od/productkeysactivation/tp/topkeyfinder.htm
During a new installation, Windows will want to connect to the Internet to validate the product key and temporarily enabling an Internet connection during the install will mean that you won't have to suffer subsequent reminders.
Microsoft may detect that Windows is being installed to different hardware necessitating a phone call to Microsoft Support. Assuming the wait in the queue isn't too long, this is usually a fairly painless experience once you explain what you want to do and they flick the appropriate switch.