The machine I am trying to install on is Windows 7 (64-bit) and ModelSim 19.3 is supported on Windows 7. Can anybody help?
I am not sure if there is Modelsim 19.3 version, do you mean Modelsim for Quartus 19.3 Pro? Anyway I suggest you to re-download (your desired version) it, it might due to the SW is not downloaded completely.
Note: QP Pro 19.3 and 19.4 use same version of Modelsim which is 2019.2/19.2