Umm...
 

I had just found out that my 10 year old sister is looking at porn on my computer...(ewww) How do I block websites on an admin account (Windows Vista)?

Slap her in the face.
That about sums it up.

man, that would freak me out

dude i found my 11yr old brother watching 2 girls 1 cup

man the worst thing that ive ever found like that was a picture of a guy i know wearing a dress in the basement in my old house

You can't block out websites with just windows. You can through your browser using the tools menu. It isn't really to good because it blocks based on rating. If the site isn't rated, it doesn't get blocked. The best way is with third party software. Programs like this Parental Controls: Safe Eyes Software at InternetSafety.com let you block based on certain criteria and block or log chat sessions. There is a lot of software that can do what you want, but I don't know of anything integrated into windows. They do charge for the software, but you can bounce around using free trials.

she was gonna look at it sooner or later :-/

That's how kids grow up, don't discourage her just get her a joint.

It's my ****ing computer!! I honestly wouldn't give a **** if she did this if she had her own PC...

Your sister is a freaky chick.