Is installing a pirated copy of Windows Vista, using an original Windows Vista license key, considered permissible or forbidden (halal or haram)?

This is determined by the company's terms and conditions. If the company allows copying the software for those who own the original activation key, then there is no harm. Otherwise, it is not permissible, as it constitutes an infringement on the rights of others. Some scholars permit copying programs for personal use, provided they are not used for commercial gain.