Trojan? computer cant do anything!

Hello,

my husband downloaded an infected file to our computer. the computer is a few years old and uses windows xp. there was no fully installed anti-virus/security on it yet, but it has windows defender and an outdated mcaffee anti virus which all alerted us that there was an infected file that could not be deleted (i think i saw the word trojan). the computer turned off by itself (logged off and everything) and i started it back up and was going to try to install the norton product we have (via disc). once it logged on it looked different though. the toolbar at the bottom looked like it does when its in safe mode (basic grey toolbar). all of a sudden i cant access the internet or windows defender and the norton cannot be fully installed. i also cannot access system restore (even in safe mode).

i dont know how to find and remove the problem, especially since i cannot access the internet to download anything.... has anyone else had this happen? any advice will be greatly appreciated!