Not sure if I use the right names for the functions I recommend, due to the german windows version I use, things named slightly different.
Tried restarting Windows? Not just off and on, since this tend not to do a complete restart unless you're changing setting within the Energy Options. It's just saving the actual state of windows to allow a quick boot.
Monitor/Software Drivers Installed? Never had problems with this, but I never used a DELL Monitor
Try a resolution that worked before, I guess you changed monitor?
Being more specific about your system might give me a hint. Processor, Audio Device, connected USB Devices.