Which NDURE - Krayzie, Kingroach or SID?
If you have a shadow C (i.e. your current C partition is empty) then you just use your current dash to manually copy the stock C files from Slayers (system\all\C) to the empty C partition on your Xbox. If you don't have shadow C then things get more complicated. Knowing which sofftmod you used will help.
However, to change the video mode you will need to disable virtual eeprom, otherwise the changes won't stick. You do that differently for different softmods, so you need to know which softmod you used. If you are trying to enable HD then you might also want to read this guide