Titan quest invalid cd

[dipdo1]

I have tried to read the forums and understand what i need to do, but alas Im lost. I got the cureroms, asr and others. I dont understand what u say by cmd line. I can play SP fine, would like to try out MP...keep getting invalid cd key. Using DT 4.06.....and TQ version 1.20. If someone could break in down step by step in plain english for me it would be great.

system:
amdx2 4400
2g ram
2 gtx 256mb 7800(sli mode)
ac'97 sound
XPSP2

Thx