WTF:Windows CD Key

i think its home edition…my computer took a shit… i built it myself its alright i got like a 300 graphics card by ATI a nvidia mother board n 2 gigs of ram…i had like a 80 gig hard drive but its fucked the virus i got like killed it you cant re format it or anything so i bought a 100 dell off of ebay n took the hard drive out of there n put it into myne thinking it will be faster n better because my specs are like 100 better than the factory dell… i got everything running and i signed on and now it says to activate windows enter in the CD KEY