Retro Virus
by Jeff
Image text: He says this is the year of Linux on the desktop! The world of Windows will fade any moment now!
The image text is a continuation of the comic and an excerpt from Howard Dean's Friendster from 2003.
In the comic, when he says that he is cleaning viruses, the other characters in the comic list off other things from 2003, including Kazaa a P2P filesharing app that was the number 1 source of viruses because of the questionable sources of all of the files on Kazaa. Additionally, Howard Dean, was popular in 2003 when he was running for president in the Democratic primary before his unfortunate screaming speech which drove him down in the poles and out of the race. Lastly, there is Friendster, which was extremely popular in 2003 and is still around today, but I'm not sure anyone uses it.
If viruses are so 2003, spyware is so 2010.
Experiment
by Jeff
Image text: The other two are still lost on the infinite plane of uniform density.
Let's see if we can work through this one. The Black Hat character and a female accomplice knock out a Physics professor and put him in a frictionless vacuum. The professor wakes up and tries to talk but the vacuum prevents him from being able to say anything. In the first two frames of the bottom section, the professor tries to move, but falls down because the vacuum is frictionless as well.
EDIT: Please see the comments below for Rob and Paige's explanations of the comic who are much more attuned to physics jokes than I am. Thanks Rob and Paige!
Prudence
by Jeff
Image text: Moments later, the White Witch rolls up and, confused, tries to tempt the probe with a firmware upgrade.
This comic is a reference to the book The Lion, The Witch and The Wardrobe by CS Lewis and later made into a movie. The children in the book are playing "hide and seek" in an unfamiliar house when one child hides in a wardrobe. They then find a entire world known as Narnia behind (or inside) the wardrobe.
The comic is making a play on the Narnia tale, by instead of the child exploring the new world herself, she sends a probe. This is similar and possibly a reference to the probes that NASA deploys to gather information about new worlds. It also may be a reference to Stargate, in which the Stargate Command sends a probe to new worlds to gather information about the world.
EDITED TO ADD: The title appears to be a reference to the fact that the child in the comic does not run blindly into the new world, like in the book, but instead she deploys the probe to gather information first, which would be seen as "prudent".
Nachos
by Jeff
Image text: 'Cheater!' 'Hey, gaming on wifi? You have only yourself to blame.'
This is another unclear conversation from xkcd right off the bat. The girl with the white hair calls the house of the girl with the dark hair and the guy. The guy answers and says that "Megan", the girl with the dark hair can't come to the phone because she is gaming. The girl in the white hair knew this already because she was playing against Megan. And probably knew that Megan was gaming over wifi.
Microwaves and wireless routers both transmit at the 2.4GHz range. In the case of microwaves, they use that range to cook food. If some of the waves leak out of the microwave, which they invariably do, they will interfere with a nearby wireless router.
And I agree with the comic wholeheartedly, why would you game over wifi anyway?
NOTE: If you haven't headed over to xkcd.com and witnessed the homage to Geocities that Randall has created, you are missing out.
Bag Check
by Jeff
Image text: A laptop battery contains roughly the stored energy of a hand grenade, and if shorted it ... hey! You can't arrest me if I prove your rules inconsistent!
It's never a good idea to mention what possibly dangerous stuff you have in your bag at the airport. Probably better off just buying a new water after the check point. No matter the inconsitency of the rules. And the joke in the image text is that you can very easily be arrested on inconsistent rules.