Downgrade Windows 8 pro to Windows 7 Pro Dell Precision T1700

1. I recommend against doing that. Windows 7 is unsupported and insecure. It does not receive security patches, so it should no longer be used on a PC connected to the Internet. Even Windows 8 is no longer supported. If you have 8.1, that is supported until January. 

2. I would suggest that you upgrade to Windows 10. Then you would have a currently-supported OS, and have many improvements.

3. If you decide to move to Windows 7 anyway, it would be best to perform a clean install of Windows. You can obtain drivers here:
Support for Precision T1700 | Drivers & Downloads | Dell Canada

About 1 variant, this machine not use internet only special software and hardware who work only on win 7, because I ask have or not legal key from dell to install win 7 pro, have I license win 8 pro, thanks for your answer's

Hello, thank for your recommendation but for this need to buy new license, I looking for hove solve the broblem have license win 8 pro by downgrade.

Just to add to the discussion... if you have software that requires Windows 7 to run (and won't run in compatibility mode on Windows 10) you may be able to install Windows 10 on the T1700 and run the software in a Windows 7 virtual machine, using, for instance, VirtualBox. You can cut the VM off from the internet and use files and devices shared by the Windows 10 host.

Hello, thanks for you answer's, about 3 variant, which key I can use to install win 7 pro? It's legal move downgrade?