Virus Types.

Hey how is everyone today, I have a general question that possibly someone can clear up for me.
I was wondering what the difference in symptoms one would receive from different types of malware.
For example. What adverse effects does a worm give the system, because no one suspects a problem until something is either slower or not in working condition.
So 6 Questions.
What adverse and noticeable negative effects does a system undergo when a Worm is present?
What adverse and noticeable negative effects does a system undergo when a Trojan is present?
What adverse and noticeable negative effects does a system undergo when a Dropper is present?
What adverse and noticeable negative effects does a system undergo when a Rootkit is present?
What adverse and noticeable negative effects does a system undergo when a Backdoor is present?
Is it all low speed that shows a virus problem?
Personally wondering because some samples I have been testing run with no process in task manager window, no visible registry, and no ill effects. But come back as an infected item right in the system folder which of course is more then likely normal, and some take names of safe files, but what is it that raises someones suspicions that virus activity is present on their machine?

maybe you find your answers here

Computer virus information and help http://www.computerhope.com/vlist.htm
Threat Encyclopedia http://about-threats.trendmicro.com/
Securelist http://www.securelist.com/en/
Virus Encyclopedia http://antivirus.about.com/od/virusdescriptions/l/blency.htm
Microsoft Malware Protection Center http://www.microsoft.com/security/portal/