Do you have Windows 10?  (The last Win 10 Anniversary update broke all of the old/non-supported versions of QB, so even if you get it installed you can almost definitely not get it open).  You’ll need to upgrade.  QB 2017 is now out.

Resolution for 'almost definitely issue: 'trying to install QB Pro 2015 on my new computer and it is not accepting my product key that is valid on my old computer. What do I do?' available: Yes.

The almost definitely issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the almost definitely issue yourself by looking for a resolution described in the article. If it is a complex almost definitely issue or you are unable to solve the issue, you may contact us by clicking here or by using other almost definitely support options.

For more information about resolving issues related to almost definitely in Quickbooks/QBO or to get support for the issue 'trying to install QB Pro 2015 on my new computer and it is not accepting my product key that is valid on my old computer. What do I do?', click the link below: