My current system is a
P4 2.6Ghz
1Gb of Memory
ATI 9800 Pro 256Mb video card.
As you can see this is by no means a 'games' machine
Thats said, I also play SHIII in windowed mode with these additional applications always open...
Outlook
Netscape
And often with Word, excel and Trillian open.
As you have a nVidia card, this may be your issue. I will get back with a framerate shortly after I sink this merchant ship and do so surfaced.