Mounting Computer Under Particle Board Desk
While there's no problem mounting things to particle board as others have described, 1/2" is just too thin to work with. I would mount another board under your desk using construction or wood glue, at least doubling the current thickness of the top. 

After that has set up for a day, I'd mount with 3/4" screws (or longer, depending on final thickness), properly piloted. 

I've mounted many things to 3/4" particle board from below, including dual coat hooks that my kids have used for years. 1/2" just doesn't offer enough grab. It's likely that the screws will pull out a layer of material when it all comes crashing down.