For those experiencing this issue, here's how I resolved it:
The user had Revo Uninstaller on his machine, so this is needed for this to work! Safe mode is not needed for the following procedure!
1. Start the machine as normal, and login
2. When prompted with the critical error, ignore it (Drag it somewhere so it remains out of the way)
3. Press Ctrl-Alt-Del, and choose Task Manager
4. In task Manager, if you don't see any tabs, or menus, tap/click "More details" at the bottom
5. In Task Manager, tap/click the file menu, and choose "Run new task..."
6. enter "C:\Program Files (x86)\VS Revo Group\Revo Uninstaller\Revouninstaller.exe" (Including quotes!)
7. Place a check in the box that says "Create this task with administrative privileges." and tap/click ok.
8. In the list, double tap/click "Avast Free Antivirus" and tap/click YES on the uninstaller prompt
9. When prompted to select an uninstall mode, choose Advanced, and tap/click Next
10. When the built in uninstaller for Avast appears, simply close it with the X in the upper right corner. (Do not tap/click continue!)
11 In Revo's Uninstall window, tap/click next, wait for it to finish scanning for leftovers, then tap/click next again.
12. For found leftover registry items, tap/click "Select All" then tap/click "Delete" then tap/click YES to confirm, then tap/click next.
13. For found leftover files and folder, repeat step 12
14. Tap/Click finish, close Revo Uninstaller, and Task Manager, then tap/click "Sign out now" on the critical error window
15. From your lock screen, restart your computer.
16. After a successful login, you can either install a fresh copy of Avast, or another antivirus product of your choice.
To avoid this problem in the future, both me, and my friend have opted for a different antivirus solution for use in Windows 10, until this bug has been properly resolved.
Thank you for your time!
EDIT: Fixed the incorrect step number referenced in step 13.
