If you strictly need the old, unpatched version, the portable option remains the best, risk-isolated approach.
Need IE8 for legacy testing or an old intranet site? Download a fully portable version of Internet Explorer 8 for Windows 7. Free, USB-ready, and no admin rights required. download internet explorer 8 portable windows 7 free
On Windows 7, you can actually turn IE8 on or off via the Windows Features menu: If you strictly need the old, unpatched version,