If you would consider upgrading it to Windows 7 you will get parental controls that are simple to control. You can set allowed access times, game access based on ratings, and which apps the child can/cannot run.
Set the machine so it can only browse pbskids.org, which should keep him busy for hours. If you do this at the router level then you don't have to worry about the kid being able to defeat the filter.
I have a 12-yr old with autism, and as soon as he was curious about computers (5 yr old or so), that's how I dealt with it. My only beef with parental controls is Youtube, which refuses to use proper ratings. Instead it takes into account the age (minimum 13) of the account holder and it refuses to show content flagged as objectionable if the viewer is not at least 18. The problem is that the child eventually runs into content that hasn't been flagged yet, which is annoying as hell. Except for the youtube issue, pretty much everything else respects the parental ratings. Firefox for some reason didn't like to play nice with these in OSX 10.5 and 10.6, but Safari is fine. I haven't checked if this is an issue with Windows 7 too.
The other thing I did is I setup his mac in the living room, facing a wall. My wife keeps her laptop at a spot in a living room that is in a direct line of sight of his, so all she has to do is look up and she can see what he is doing.
And yeah, *my* laptop is set so I can see HER screen and his. She's clueless about computers, so I have to keep an eye on hers as much as the 12-yr old.