While malware can originate from porn sites, it's not a given it will be on your computer after viewing porn sites.
Since all the scans you ran came up clean, I think it's safe to assume the system is clean.
If you want a little more peace of mind, download and install
Spyware Blaster. Update it immediately and enable all its protection.
If you use the freeware version, you'll need to remember to update it manually...I check once a week. It's quite good at keeping spyware from getting into your computer in the first place.
Another way to avoid spyware is to use the
Firefox browser instead of Internet Explorer. It hasn't yet been exploited by spyware writers to any noticeable degree (although that may change as it gains in popularity).
You'll need to keep Internet Explorer but you can delete shortcuts to it from the desktop and Quick Launch bar, substituting shortcuts for Firefox, for the next time your "friend" wants to cruise porn sites on your computer.