Title text: Viruses so far have been really disappointing on the 'disable the internet' front, and time is running out. When Linux/Mac win in a decade or so the game will be over.
Cueball shows off his virtual fishtank of virus-infected virtual Windows machines to Megan. The machines nominally have mail trojans, Warhol worms, all sorts of polymorphic viruses, and explicitly Blaster and w32.welchia. Cueball relates to the viruses as though they are fish, and hopes that they are all getting along together nicely. This is because part of welchia's malicious payload was to remove the Blaster Worm , effectively destroying it.
It would be possible to set up a virtual fishtank as described. The main issue would be to make sure that you don't accidentally let anything escape from the fishtank. Consider it like a smallpox lab. Also, some viruses are quite malicious  and will prevent a computer from running normally, or at all. An aquarium of dead computers would not be very interesting to watch.
The first part of the title text refers to the difficulty viruses have in the common doomsday threat of "disabling the internet" as a whole. SQL Slammer had some brief success. The second part of the title text indicates that Randall believes A) that Linux and Mac OS X are inherently less vulnerable to virus attacks than Windows, and B) that Windows will become less important and disappear, so the virus writers had better get their act together soon.
It is not certain how justified this opinion is. Six years after this comic was written, Windows still dominates the desktop, and Linux and OS X are not that much harder to attack with viruses. A side issue is the wild growth in 'smart devices' connected to the internet, powered by non-traditional operating systems such as iOS and Android. Desktop operating systems such as Windows, Linux and OS X are all becoming less relevant (although note that Android is based on the Linux kernel and iOS is based on OS X), so both the operating system war and the struggle against computer viruses are still "anyone's game".
Something similar to this is now online (as of 02014-06-29; not sure when it first started), with some differences from the system described by this comic. Instead of executing email attachments, the 7 VM's run files uploaded to http://wecan.hasthe.technology/ by the public, making it more of a public playground aquarium than a private fishtank. Instead of wiping machines at random, each VM runs a virus scanner every 24 hours.
- [Megan looking at a large screen with many green and red squares. The squares have writing in them and lines connecting them.]
- [Side view. The screen is a huge LCD connected to a wireless router.]
- Cueball: Pretty, isn't it?
- Megan: What is it?
- Cueball: I've got a bunch of virtual Windows machines networked together, hooked up to an incoming pipe from the net. They execute email attachments, share files, and have no security patches.
- Cueball: Between them they have practically every virus.
- Cueball: There are mail trojans, warhol worms, and all sorts of exotic polymorphics. A monitoring system adds and wipes machines at random. The display shows the viruses as they move through the network. Growing and struggling.
- [Cueball walks past the girl and touches the monitor.]
- Megan: You know, normal people just have aquariums.
- Cueball: Good morning, Blaster. Are you and W32.Welchia getting along?
- Cueball: Who's a good virus? You are! Yes, you are!