The one question not asked is what restrictions are we talking about?
I would guess we're not talking about a local game, as there are few that would work on Windows and *nix.
There are the time limits, but I ask again what is the purpose of the time limits - so I guess we're really talking about limiting time on the internet and/or access to certain websites.
Working on that assumption (I know, the problem with assumptions...) You could do a lot of the access controls already mentione. (I do like Iszi's suggestions best), but maybe the issue is that we're not looking at the right level to add controls.
If your end goal is to limit access to the internet in general and/or certain specific sites, you may want to move up to your router's access controls and/or use a different DNS service like OpenDNS.
Most modern routers will let you enforce time limits and access restrictions based on the MAC address. This way it's based on the NIC and it doesn't matter what he boots into. There is one little footnote I should add to this option - it is possible to spoof the MAC address. Ok, there are two footnotes I should add - he could log into the router if you do not have a good password in place. For the first footnote, there are other questions that cover that issue in more depth than I have the skill to cover. As to the latter, look at Steve Gibson's Password Haystacks page and listen to the linked podcast.
Services like OpenDNS (I only mention that one in particular because that is the one I am familiar with) will let you enforce content filtering at the DNS level. Your child would have to know how to manually set up an alternate DNS to bypass that. Not impossible, but more effort than booting up a Live CD.
Even after implementing one or both of the above suggestions, you still need to look at the parenting end of this issue. (again, back to Iszi's answer) There is no such thing as a hack-proof system - any security one person can set up, another can find a way around given time and access.
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