How long can computer remain running?

It will run till it dies! Most often a good computer can run till it cant be upgraded or the parts cant be replaced/too costly to do so. I have a computer that I still use every once and a while just for old time sake The CPU is a 300 MHz (GHz CPU's weren't yet invented) EDO ram (bet you haven't heard of them either!) It started with Windows 3.1 and over time was upgraded to windows 98 but couldn't take XP! Like I said it still runs like a champ! IF you maintain your machine it should run for many years. Most people upgrade or buy a new machine because of the newer spec. or OS..long before the machine passes beond the "usefulness" stage!