basicly you can, but you have to take into account, that a notebook is not build for that sort of duty. i use my notebook very heavy, its on allmost every day for 10 to 18 h, and nothing went wrong with the hardware.
if you intend to use it as a server, my advise is to buy a specified server, if you just want it 24/7 for filesharing, try it, in my expirience most of the fileshare progs tend to crash after a week or so and you have to reboot anyhow.
