If you run into the Steam main exception error Unable to load library SteamUI.dll, here’s what you can do.
1. Step Many Steam errors can be fixed by restarting your computer. I can’t stress this enough, because it really helps.
2. Step If the error persists, go to C:\Program Files\Steam\ or C:\Program Files (x86)\Steam and move all files in there (not the folders!) to a folder called backup EXCEPT Steam.exe, then restart Steam using that .exe file
Here is a list of files that I moved:
After moving the files you might get the error Steam.exe main exception: ERROR: copying SteamNew.exe to Steam.exe failed Win32 Error 32 “The process cannot access the file because it is being used by another process”
If you get that error make sure you closed all Steam processes using the Windows task manager (CTRL + ALT + DEL), go to tab Processes and enter S and then right-click on Steam.exe to kill it (End process)
Should you still have problems, it helps to rename Steam.exe to Steam.exe 2 – then start Steam via Steam2.exe, kill off the Steam2.exe process and then deleting all files again (only a few will be in the folder). Finally, you have to rename Steam2.exe to Steam.exe again and it should work.
3. Step If the error persists, try to disable your firewall and anti viruses temporarily to see if it can fix the problem.
4. Step Still no luck? Uninstall Steam and re-install it.