Make .pfx from .cer and .key on Windows 10

The easiest way I found to do this on Windows 10 is with certutil.exe.

1. Make sure your .cer and .key have the same name e.g. domain.cer, domain.key and put them in the same directory, I use C:\cert\.
2. Open an Administrative Command Prompt and make sure you are in the C:\Windows\System32 directory
3. Type this command >certutil -mergepfx C:\cert\domain.cer, C:\cert\domain.pfx (the command assumes domain.key)
4. Before the command completes you will be asked to enter a password which you should do because most certificate upload routines demand a password protected .pfx file.