Most laptop user manuals I've seen cover how to replace the hard drive. If it didn't come with one, you should be able to get it from an Asus site somewhere. Since they're global, there may be regional variations. The manual will also cover things like which keys to hold down and when to get into the BIOS setup, and probably how to do a system restore. If the hard drive crashed you'll definitely have to do a system restore from DVD's, since it will have all the drivers for your hardware, which is very specific and proprietary on laptops. So if you don't have those, that's something else you'll need to get from Asus.
I've only owned and worked on a handful of laptops, but they all used some sort of bracket that screwed to the hard drive, then the assembly screwed to the laptop chassis. But Asus is a top tier company, so if they refurbished it and it doesn't have one, who knows. Except the user manual should tell you.
If its supposed to have a bracket but doesn't have one, and doesn't get moved around much, you might try some newspaper or something between the cover and hard drive to wedge it in. I've done this on a temporary basis when cloning hard drives and didn't have any problems. The best solution of course would be to get a replacement somewhere.
Albert