1 - When you close it at night, you can tell it's still on for some reason.
2 - When you open it in the morning, it's really slow to start up. It might even refuse
3 - The mouse doesn't track when you move it, but has a 10-second delay.
4 - At random times (esp. late at night) it consumes large amounts of battery power for no reason (i.e., "munchies")
5 - It starts hanging out in unsecured wireless networks.
6 - You start finding torrents on the hard drive.
7 - It can no longer multitask. When you alt+tab, the previous application freezes.
8 - ctrl+alt+del no longer works for minor problems. You take it in for serious professional help.
9 - All the names of your iTunes songs are suddenly replaced by scrambled Beatles lyrics (really happened)
10 - Your screen randomly changes to a ridiculously small resolution.
11 - It has trouble controlling the volume of its speakers.
12 - Every few months it has another complete meltdown, requiring a reformat.
13 - It refuses to run any application (not even Notepad) but video games work fine.
14 - You notice a weird "baked" smell emanating from the back.
15 - It starts arguing with itself. Symantec puts the latest Windows update in quarantine, so Windows downloads it 3,000 times.