J.N.
You will get a lot of suggestions about great software, and I would definitely go with one of them. But don't forget that NOTHING replaces the vigilance of a parent!!!!
The computer MUST MUST MUST stay in a public area! Start now and when she's a teen it won't be an issue. Always watch what she is doing when she's on the computer. Talk to her about what to do if she accidentally gets to a site that is not okay (close the browser immediately) - becuase some people are really jerks and will work hard to bypass any software - and someone will succeed. Start talking to her now about good-touch/bad-touch (this is not specifically computer related, but it is a natural lead in to if she sees images). Keep it age-appropriate, but she needs to hear it (they start teaching it in school as young as kindergarten, and even preschool, too). If you arm yourself and your daughter with knowledge of what to watch for and avoid, and what to do when it does sneak in, you will be helping her for years to come in safe internet practice.
Find the sites you want her going to and save them in her favorites, then teach her how to find them there, rather than trying to type in the URL. It'll get her there a lot more quickly and avoid her accidentally going to the wrong place.
And here's a great website to take her to early on: www.starfall.com . It is a letter recognition/early reading skills program that all my kids (my own and the 80+ kindergartners I have taught over the years) have really liked. Enjoy!!