Hi, I have found the solution for the common "Error 1603" problem that occurs whenever we try to install Solidworks 2015 SP 4.0. Please find the attachment. Enable windows installer service (If you can't see it, type "CMD" in run and inside the command prompt, type "msiexec /unregister" and "msiexec /regserver" and restart to find the "Windows installer service" in the "services.msc"). type "services.msc" in Run and enable windows installer service. Disconnect from the internet and install!. Works perfectly!