STEAM error

I just started up an old gaming pc I got from a nephew. It's a windos xp.
When I try to play PA on this steam gives me the error: "Game Failed to start (unknown error)"
Is this because it is an xp or is there an other problem?

Unfortunately PA supports only Vista operating system or above, so you won't be able to play on Windows XP.

Ahw owkey I thought it was something like that. Thanks

You're welcome! ^^

Though if you have 4GB RAM then hardware itself will handle game on minimum settings just fine.
It's really old, but still support all features game need.