What should be done after running an unknown script on the computer?

[Charade]

Hello, a friend I trust a lot has made a script and asked me to try it out. I tried it, and the Command Prompt (CMD) opened and was typing something like 8 bytes. There were around 87 million files, I think. I can't remember exactly, but I'm concerned it might be trying to steal my cookies or something. I closed CMD, ran a virus scan, and found no results, but I'm still unsure. How can I make sure?