Virus-like program that spreads automatically to other computers by sending itself out by email or by any other means. A program that propagates itself by attacking other machines and copying itself to the affected machine.
Worms have self-replicating code that travels from machine to machine by various means. A worms first objective is merely propagation. Worms can be destructive depending on what payload they have been given. Worms may replace files, but do not insert themselves into files.
W32.MSNmaker is a worm that spreads through MSN Messenger. This worm arrives as a file manually sent by a malicious user via MSN Messenger.
When executed, this worm rename the original file as msrr.exe and replaces the original MSN Messenger executable file msnmsgr.exe with a copy of itself. When the worm is executed, it also executes msrr.exe to avoid immediate detection.
A typical snapshot showing the chat message containing malicious link sent by MSNMaker.