Reset display settings for a monitor that is not plugged in?

I'm running an ASIS G73S laptop with GEFORCE GTX440M in it. I wanted to hook it up to a nice 27" LCD Dell U2711 monitor which support 2560x1440 resolution. Unfortunately, there's no DVI or DisplayPort outputs on the laptop and the monitor doesn't report any resolutions above 1080p as available via HDMI. I added a "custom resolution" via the nvidia control panel which works ... until my computer powers off or sleeps. Once it comes back the system locks up and the screens don't come on until I unplug the external monitor. After that, plugging in the external monitor will lock up the system with blank screens.

Any idea whether I can get windows and the nvidia control panel to "forget" the custom resolution on the external monitor? Ideally without reinstalling windows?

Any help appreciated!

I found a way to do this, I deleted all the subkeys of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers in the registry that had DELL in them and this made windows reset everything to default next time I plugged it in. Yay!