Problems with graphic target (maybe, not sure)

I give up. I tired everything and now i ended writing in the official forums, im out of ideas, i hope you can help me ._.
My girlfriend and me have been playing COX for 8 monoths or so, with a good new computer which used vista and a ATI 3400. We never had problems, and we always could play in maximum details. The problem is we hate Vista, so one week ago we went a computer shop and payed a technic to install my original Windows XP in our comps.
When we got home, first of all we installed COX. Surprise? Downtime or lag. I checked everything: internet conection, ATI drivers, a possible virus, DirectX, even uninstalling programs like SRS sandbox, reinstalling COX, testing in all servers... as i said, im out of ideas.
Why we have a graphic lag (i dont know how to call it better) with XP, if we were running properly in Vista? I have the latest driver from ATI website, so...
Ah, one more thing, when i set the 3D resolution scale and graphic quality to the lowest i dont have any problem (obviously i cant play at that way, it's ridiculous). The only way i can play it's in 800x600 resolution and minimum detail, but im not going to keep playing if i cant solve this... and i dont want to leave the game, i have lot of fun, so any information that someone could give to me it will be VERY apreciated.
~~ Sorry my bad english =_= i hope at least someone understands what im trying so say.

Hi,

If you haven't already, try turning on "Advanced Settings" in the Graphics options, then turning down the settings for the following:

“Water Effects”, "Desaturation Effects" and "Bloom".

Start with the first, as this being set to "High Quality" has been known to cause performance problems on some systems, and setting it to Low or None is a common solution to the problem you describe.

Hi, thanks for replying so fast, GhostRaptor!
Unfortunatetly, my problem keeps chasing me. Since turning on the lowest detail level disables those three options, i tried it already. What i did now, is using higher detail options (performance laged me already, i have to add), but disabling those three ones, whith no luck at all.
Seems like if i still want to play, i'll have to do it in 1024x768 with low detail, the best appearence i can play with no problems *sigh*.
I still have the hope someone had the same problem and could solve it... somehow.

Only thing I can suggest, then, is to contact the support team. They may be able to help you track down the problem.

another thing you could try is turn shadows off, i have a similar graphics card and the game was extremely laggy, but by turning them off it now runs fine even with just about everything else at the maximum.