You must have clicked something he sent to you

You must have double clicked on something he sent to you. This incident may not have to be harmful to you, but in the future, when you receive something from other users, there are several recommendations:
1. Do not double click on any executable files, especially any files that request UAC.
2. Try not to download any software and try to install it on your machine, unless it is from well-known source. For example, if you want to install Acrobat Reader, download it from Adobe.com, don't just download it from other websites, they may be buckled with malware.
3. Do not turn off UAC. It may be troublesome at first, but it will be benificial in the long run. And most of the time, you will only need UAC when you install software.
4. Try to run everything with stardard user account. If you think one user account is compromised, delete that account and create a new stardard user account, and start to use the new one.

You can use windows defender to find out what processes or services will automatically run when your machine starts up or when you log-in. See which one you are not so familiar with. Also you can open Task Manager to see what processes are running.

If you really worry about the security of your computer, reinstall your computer, and follow the recommendations.