How Trojan Horses or Worms affect a computer system? What guidelines should be adopted to troubleshoot along with malicious software?

Trojan horses are stand-alone programs. The look like what they are not and appear to be sometime interesting and harmless (e.g. a game) there execution results in destruction.

Worms only create copies of themselves on the infected computer.
Following guidelines should be adopted to troubleshoot along with malicious software:
• Download SW from trusted sites just.
• Do not open attachments of unsolicited eMails.
• Use floppy disks and CDROMs that have been used in trusted computers only.
• While transferring files from your computer to another, use the write-protection notches.
• Stay away from pirated SW.
• Regularly back your data up.
• Install Antivirus SW; remain it and its virus definitions updated.

b) Which part of a virus consists of malicious computer instructions?
Payload is the part of virus in which commonly consists of malicious computer instructions.