Magic CommandsOften, a user will paste a seemingly benign piece of code into a public channel, under the pretence that it will "STOP SPAM" or "OP YOU ON ALL CHANNELS".
Naturally, what these pieces of code really do is infect your client and compromise your security. Possibly, they may even send your NickServ account details to a specific user or channel on IRC. These sometimes appear in private messages on join as well.
Often, these commands will begin with //, and may contain a $decode. Do NOT use these unless you know for sure what it does.
These pieces of code will probably infect your client with a malicious script that sends the above message to everyone who joins a channel that you are on, infecting them as well if they run it. They may also give anonymous users remote control of your client, and allow them to perform commands without your authorisation.
Suspicious URLsThese are usually leading to executables that promise to improve your ping in online games, or any number of other wonderful things. Do NOT click unless you know for sure where it goes.
A good rule in the handling of URLs is never to click links on IRC unless you are sure of where it takes you. Do NOT blindly click links from other users you trust, as they could be infected with a worm. If they really sent you a link, as opposed to a script/virus sending it, ask them what it is first, as they can respond to you, a script/trojan won't.
Remember that a link to any kind of a URL -may- be malicious.
A free and easy-to-use online virus scanner (Housecall) can be found here.
You should have some sort of anti-virus software installed on your computer and scan for viruses/trojans on a regular basis.