easiest way is to use C for your o/s & programs and D for your data. So if your o/s gets knackered you can simply re-install windows without it causing problems for your data.
If you have two separate drives (rather than one hard drive split into two partitions) then also make a folder called 'Back-Up' on your C drive and make a copy of all your data stuff. This would be an added
precaution - so if your D drive died a death.