Where is adb.exe in windows 10 located?

You can find it here:

%USERPROFILE%\AppData\Local\Android\sdk\platform-tools

To save yourself the hassle in the future, add it to your path:

1. Click 'Start'.
2. Type 'Edit environment variables for your account', and click it.
3. Double click PATH.
4. Click 'New'.
5. Paste that path in.
6. Click 'OK', click 'OK', and restart any command prompts you have open.

It is located in the AppData hidden folder

C:\Users\[user]\AppData\Local\Android\sdk\platform-tools

From l33t's comment below you may use the following shortcut:

%LOCALAPPDATA%\Android\sdk\platform-tools

To use it in PowerShell you can do: $env:LOCALAPPDATA\Android\sdk\platform-tools