FPS problem ET vista :<

Hey, im having some hardcore fps lags all the time. Its not drops, but im just laggin when I see many details in the game. I have tried to set down my graphics ingame to the lowest, but still im lagging like hell. Im using com max 76 fps but my fps is around 40-60, and it gets even lower when I am on big maps. I am also getting lags when I shoot!

I wanna "downgrade" my vista, but I lost my vista cd so i cant format it, and install xp. :(

If any1 can help, or know what to do. I would be very thankful if you'd contacted me asap.