According to a biannual Security Intelligence Report from Microsoft, AutoRun -- the feature in Windows that automatically executes files when you plug in a USB or connect to a network -- accounts for almost half of all malware infections. That's really damn high.
To be clear, these are infections that don't require any user-input from you, so it's kind of not your fault that your computer gets infected. By turning off AutoRun, you'll add an extra step to certain tasks, but it's worth it to cut down on malware 50 per cent.
What's also interesting in this report is that Windows XP SP3 systems get infected about ten times as much as Windows 7 SP1 64-bit systems and six times as much vs. 32-bit Windows 7 systems. That alone is one reason why you might want to upgrade your parents' machines to Windows 7.