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.