We would like to inform that the Intel® PRO/Wireless 3945ABG Network Connection reached End of Interactive Support on December 2013, Intel no longer keeps drivers for it at the Download Center. You can check the https://www.intel.com/content/www/us/en/support/articles/000006507/network-and-i-o/wireless-networki... Customer Support Options for Discontinued Intel® Wireless Products for details about EoIS products.
Your best option would be to obtain the driver from your http://www.intel.com/content/www/us/en/support/topics/OEMs.html Computer Manufacturer Support.
In our best effort and in case the manufacturer doesn't have the driver either, you might want to try the following options; unfortunately, we don't have a system like yours to test, so these may or may not work:
1. Try using https://www.intel.com/content/www/us/en/support/intel-driver-support-assistant.html Intel® Driver & Support Assistant and check if it detects your adapter.
2. Try downloading https://downloadcenter.intel.com/download/27239/Wireless-Intel-PROSet-Wireless-Software-and-Drivers-... Intel® PROSet/Wireless Software and Drivers for Intel® Centrino® Advanced-N 6205, file: WiFi_Win7_64_19.70.0.exe. This download is meant for a newer adapter, however, the tool may be able to pick up older adapters such as the 3945.
Please let us know how it goes.