Hello angrychair, welcome to Seven Forums!
You do not need 'active' internet to install.
Study the information and steps outlined in the tutorials linked below to sort through each in-turn and be sure to post back with any further questions you may have and to keep us informed.
If the installer is one you acquired perhaps the ISO file is bad and another will work; or if you burned it to DVD, another burn is needed, use ImgBurn at no greater than x4 speed with a verify.
ImgBurn Free ISO Burning Software
If the PC is capable, add the installation files to a USB stick and boot that for the install.
USB Windows 7 Installation Key Drive - Create