Any other changes you've made lately?
I'd say primary culprit would be increasing your desktop size or colour depth, or increasing the graphics settings within NL itself.
If you're strapped for video RAM, I'd say stick your desktop at 16bit 800*600 and run NL at 16bit 640*480 - make sure you don't enable too many other options either. IE, Disable reflections, shadows and the like.
Let us know how it goes