It was a PDP-10 at Harvard, and it was not just Gates, it was Gates and Paul Allen working on it together. Another interesting thing is that the PDP-10 was being paid for by Department of Defense grants, and when an auditor discovered how much computer time Gates and Allen (who was not even affiliated with Harvard) had used up, there was a disciplinary hearing, after which Gates left Harvard.[1] In his "Open Letter to Hobbyists"[2] Gates claims they used $40,000 worth of computer time. So basically Microsoft started with $40,000 stolen from the government.

[1] This is documented by Noam Cohen in The Know-It Alls. [2] https://en.wikipedia.org/wiki/Open_Letter_to_Hobbyists