Trojan virus

I get Norton Internet free with Comcast.   I have been getting  Trojan Powerlik activity,   Trojan.Ad click activity,  and also High Memory Usage.   Its making my computer run slow,  and I constantly have to sign in on everything.   It also tells me  that my page has to be   shut down.     Yesterday  I ran a complete scan on my computer  and I also ran the Norton Power Eraser  and it still happens.    What do I have to do next to get rid of this virus ?    Thanks for any help.    Carol