I’m interested in the answers to this too. There are some good ideas here.

My children enjoy Starfall

Abcya and starfall are some of the best my little one is always playing them when she gets computer time they are great for lots of age groups

I suggest you check out your library; this way you can try them and see which ones best suit.

Yes, google CBBC or CBEEBIES games, they’re excellent and free

abcya.com
it has grades up grade 5 just watch out for the occasional american money game.
suessville.com
super fun and educational
and reading eggs

Check out http://www.zoowhiz.com
It’s suitable for kids 5+. A great way to learn in different areas. It’s free.

Check your local library. Maybe ask at your child’s school. They may have some idea where you can get them. Some schools actually have PC programs of some sort you can access from home. I don’t know whether or not they are free or how much they cost.