Once your computer is infected, Virus Doctor starts a fake scan and loads tons of popus saying that your pc is infected. One of Virus Shield 2009 popups says:

“System Alert
malicious applications, which can contain trojans, were found on your PC and need to be immediately removed. Click here to remove these potentially harmful items using Virus Shield.”

Both Virus Shield 2009 and Virus Doctor use scare tactics to lure user into buyin full version. It’s just another one scam tactic.