System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.36, GTX 1070 PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.36, GTX 1070 PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.36

Blender Version
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Worked: (optional)

Short description of error
Installing Blender 2.8 effectively murders my 2.79 installation.

Exact steps for others to reproduce the error
Install Blender 2.8, then try to install re-install 2.79 - Bingo!

I have paid for Addons I use in 2.79, which don't work in 2.8 yet. And now 2.79 will not install, and I shall have to un-install 2.8.
Also I need to use 2.79 for Dynamic Paint with weights - as there appears no way to use it, even vaguely elegantly, in 2.8 as yet.
And trust me: I really would rather use 2.8 vs 2.79 as I've been using it almost exclusively for the past 3 months (except for things it can't do that 2.79 still can, see above), and the interface is a dream compared to 2.79.

Update - I tried un-installing 2.8, re- installing 2.79 (in a folder edited to be 'blender2.79' rather than 'blender'), then re-installing 2.8:- 2.79 still gets killed, so effectively, every time I need to use vanilla 2.79 I will have to un-install and re-install, then do the same again to use 2.8...
Have you heard the British expression 'throwing the baby out with the bath water'?!?...