Intel doesn't have a driver for Windows 7 with this 845G onboard VGA graphics, you won't be able to get above 1024 resolution with a monitor attached. You CAN however install remote control software like TeamViewer and disconnect the monitor then control it from your main PC. (I hope you aren't trying to use a 10 year old Dell as your main PC...)
I was able to 1600x1200 resolution this way.