How can I filter my internet connection based on content?
You might want to consider using OpenDNS (http://opendns.com/), though it's strictly not based on content but on domain names.
Use Dansguardian, though I'm not sure how to set it up for a Windows PC
And we had questions like this before:
How to set-up a Linux machine for children’s Internet use?
How to help children avoid the dangers of internet.
Here's Joe answer from there (kudos to Joe):
Software4Parents.com's Top 5 Internet Safety Tips
Tell your child to NEVER EVER reveal their name, address, phone number or any other personal information to ANYONE online. Once you give out this information, it is impossible to retract.
Communicate regularly (not just once) with your child about WHAT they do online and WHO they talk to online. If you have actually met the friends they are talking to in person, you'll know it is OK for them to chat with them online.
Take computers out of children' rooms and put them into public areas such as the family room. Many parents think they are helping with homework by giving the children a computer, but it also opens certain dangers that you may be unaware of.
Choose your child's screen name, email address or instant message name wisely - don't' reveal ages, sex, hobbies, and CERTAINLY NOT suggestive or sexy names. Predators are more likely to pursue a child with the screen name "sexyteen5" than "happygirl5"
Use technology to help you protect your child. Monitoring software gives you the ability to review your child's Internet usage. Even if you don't look at each and every email or instant message they send, you'll have a good idea if they are making smart choices online.