If it boots in one computer and not in another, it may be your bios boot settings (I don't think it's a virus, but maybe you should check that).
I don't know, it could have some hardware failure, those memory sticks. It's rare that it happens in both at the same time, but it can happen. (if they suffered from same physical external aggression, no idea...)
Oh, you say it's not single pc specific. Well, I'd check if in the usb disk, if you can browse it from another computer, the config files surely of grub have been deleted or something. Also, maybe it got wrecked some how the MBR, the master boot record. It's strange that it happened in both. In Windows, it's common that viruses do infect and wreck the boot area, but...
Also, to deal with grub boot problems, you may wish to go to another computer, make a boot floppy disk, cd or usb with supergrub, it helps in fixing several boot problems, although I have not used it. When I lose boot, it happens rarely, I use whichever boot manager to recover it, and if Windows, even just a DOS command. Supergrub is here: http://forjamari.linex.org/projects/supergrub/. You can recover it in other ways, too. But this is probably easy. In case this is what happened.