Or another game console... Much easier to figure out what you need in situations like that. You buy the console, and all the software (games) are written for that console's hardware.
Not sure about the original Half-Life, which I think was a PS2 only release, but Half-Life 2 was released for the Xbox.
That's really about all you're going to find for under 500 quid anyway. As already stated, gaming requires power, and power costs money. So if that's your budget, a console or two would be a better investment. You could probably get a PS2 and Xbox for around 300 quid, which leaves around 200 quid for games in your budget. If you go to a place that sells used games and consoles, you could probably make your money go a bit further. Not that you need to spend the entire budget, since anything left over could be put away for to get a gaming PC when you've got a bigger budget.