My kids play Steam on my PC. Not all of them have their own account on the PC, but they do have their own Steam account, so I sometimes end up launching games under the wrong account (say, if I click directly into a game before launching Steam using the taskbar contextual menu).

You can create a Steam shortcut that always launches Steam in a specific account, as a workaround.

If you want to create it in your taskbar, first navigate to this directory:

%AppData%\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar

Shortcut properties:

• Target: "C:\Program Files (x86)\Steam\steam.exe" -login MYACCOUNTNAME
• Start in: "C:\Program Files (x86)\Steam"

I picked an icon that was different from the regular Steam icon, to differentiate it in my taskbar. I keep the regular one around, since the custom shortcut annoyingly doesn't have the contextual menu that the regular one has.