A new Trojan horse program was infecting PCs on Wednesday, exploiting a hole in Windows systems to sneak onto computers, then dropping adware or spyware or turning them into zombies, according to several Internet security companies. <A HREF="http://news.zdnet.com/2100-1009_22-6011406.html?tag=nl.e589">The Trojan, dubbed Exploit-WMF</A> (Windows Meta File), was rated a category 2 level risk, meaning it had the potential to continue to spread, said Dave Cole, director of security response at Symantec.