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I installed Portal2 on my system.. but it works at terrible frame rates...audio going way ahead of video (1 frame evert 2 seconds or smth like that).
switching to OpenBox and with no audio did make a bit of a performance boost but still very low frame rate.
Configuration: AMD x64 5400+.
nVidia GT 240.
2gb ram.
It ran smooth on my windows7, so it's not my pc being to slow..
Any suggestion would be appreciated.

1- Be sure to use latest Nvidia Driver
2- Lower Portal 2 graphic settings to minimum
3- Disable "Steam Community"

If after all this, it still slow I don't know why, cause it ran perfect on my PC and some friend reported it working too.

It did that for me too, occasionally. But only in XP compatibility mode. When I switched to Windows 7 mode, it runs smooth.

Interesting info, if GodsOtherHand confirm it working too, I will modify PoL installer to switch win7 mode by default for this game.

How do i switch to windows7 mode?

Right-click the game, select "Configure wine", then in the Applications tab, there is a pull-down menu near the bottom, where you can choose the version.

That said, it used to work in XP mode for me for a while, but usually after a short while of playing, I get the symptoms you are mentioning. And also some texturing glitches, which also went away when I switched to Win 7 mode. Vista mode seems to work as well. Your mileage may vary. I have Nvidia 260 with the latest drivers, btw.

Works good now. not as good as on windows, but good non the less