Restoring windows back to factory settings

I have got a Dimension 3100C, I bought a new netgear modem/router and every time i powered it up it would shut the computer down so i got in contact with technical support on there website and they told me to try different things but nothing worked so in the end they told me to do a system restore so it would be like it just left the factory. After i did this then tryed the modem/router again the computer didn't shut off so it solved my problem, so i contacted tecnical support again and told them that it work but i wanted to no why i had to restore the computer to factory settings when the computer is new and haven't had anything installed on it but they couldn't give me a answer and they just said i would have to phone software support team but its 50 pence a minute but i don't want to pay that.

So does anybody no why i had this problem.