Which plugs did you put in?
The factory fitted ones are Platinum tip or something and they're meant to last for about 100-120 thousand kilometres. They are a lot better than the standard plugs. If they have a very tiny and sharp electrode top, then those are the ones and they may look like they're rooted but they might actually still be fine.
Try and get the NGK Iridium Tip plugs next time. They are about $18 a piece but I heard they were very good.