I recently bought borderlands2 and the first time i loaded it up it ran fairly well, around 35-40 fps (playable i guess you could call it)
Now when i load it up at the 2K logo i barely get 10, at the main menu i get 50 and when i go to load into a game where it displays the gun and where you last left off i get 10, when it puts me in a game it has trouble getting over 20 fps even on lowest settings. Here are my specs:

Xeon quad core 3.2Ghz
GTX 260
2g of ram
Vista
1920x1080 (I have lowered it but it seems to make it worse)

My PC does not seem to be bottlenecking i check the cpu useage and the RAM usage, CPU stays around 40, and RAM is about 60ish.
I have no programs running in the background besides steam and nvidia settings and fraps.
Somebody please help i want it to be a good looking game and a enjoyable one aswell. Thank you!