There's two possible causes:
1) Your monitor. By default, Nvidia's control panel leaves scaling up to the monitor, so this is the most likely cause. Mess around with the monitor's menu and see how it handles scaling.
2) Your driver. In the Nvidia control panel, look for "Adjust desktop size and position" and try "No scaling". I think that "GPU" has to be selected under "Perform scaling on..." for this to work.
Of course, the best solution for this problem would be the widescreen mod