I would certainly recommend against that.
1. minimal recommended distance of reflex tube starts at about 40cm against solid material +- give or take a bit depending on size, performance etc. etc... note that side plate of computer case is rather a drum than solid metal.
2. you'll be getting rogue noises from vibrating desktop reflecting the subwoofer
3. there is possible lifespan reduction of components - mechanical hard drive, any bearings in general, soft contacts... altough this point is more academical, what would hurt most would be a sine wave (regular oscilation)... which hopefully no sane person should be playing out of any audio device
Perhaps except for hdd I'd say usability lifespan of average PC would run out long before accumulated subwoofer vibrations cause factical damage.