Not all antivirus has rootkit detection. Only some antivirus integrates rootkit detection and scanner and even if an antivirus has rootkit detections, it will not 100% detect all rootkits. However, with integrated rootkit detection, there is chance to protect/remove without having to reformat. Depends on type of rootkit though.
Use anti-rootkit scanners (manual scanners). See Marianna's post on several antirootkit at our sticky thread:
http://forums.cnet.com/5208-6132_102-0.html?forumID=32&threadID=238606&messageID=2436433#2436433 or my post at http://forums.cnet.com/5208-6132_102-0.html?forumID=32&threadID=238606&messageID=2468052#2468052
Create a standard user account (non-admin permission) to help protect from such.
I guess this is where I was told to post this. My grandson unthinkingly clicked on what he thought was a Windows update. It downloaded viruses, trojans and a hacker. He quickly ran his AVG, Windows Defender and Spybot. Spybot removed them but they returned. Acer (he has an Acer laptop) told him he could try reformat but it could be imbeded in his harddrive and he would have to buy a new one. He removed them with Spybot and did the reformat, running checks on his files before reloading. The files were clean so he is hopeful everything will be ok now. If not, I will have him explain all the details to you. In the meantime, are there not avs that will block these things from downloading in the first place? Someone suggested this is a rootkit. How do you defend against them and remove them? I know they are very nasty things. Thanks so much for any help.