ISP will always have access to everything you see ever. Nothing is sacred.
Never store content locally. Always browse in a cookieless (privacy mode or whatever) session, or delete all local content. Any videos you download (i.e. "Karasnightout4.wmv") will be saved by Windows in recent documents.
Only go to sites you trust. Porn industry is the second leading cause of viruses (next to torrents). If they let ads slip through that carry viruses, they can't be trusted. There are always other places to go.
Spybot Search and Destroy should be run occasionally to make sure the system is clean. http://www.safer-networking.org/
Every time you download a video, verify that the file extension is actually a video format.