Conventional wisdom says there are few things more useless than an old PC. You've probably got a doorstop PC sitting around somewhere, a computer that's 5 or 6 years old, with a processor that crawls, way too little RAM, a tiny hard drive, and all the wrong ports -- serial and parallel, PS-2 keyboard and mouse, where what you need is USB ports and lots of them.

What can you do with a junker like that? Upgrade it? But it needs everything. Upgrading, says the conventional wisdom, would cost as much as a new PC, so why not just buy a new PC?

But the conventional wisdom just might be wrong. Surprising as it may be, reviving an old PC is not only possible, it may be economical. There are three keys to the puzzle.

1. Are you happy with the drives and operating system you're running? Even though they've dropped in price a lot, hard drives and optical drives account for a considerable portion of the cost of a PC. If increasing the size of the hard drive or adding a DVD burner isn't something you think is absolutely necessary to bringing your old PC back to productive life, then you've passed the first test. And if you're happy with the OS installed on your PC, that removes a second hurdle. (And there's a plus: you avoid the effort and risk of reinstalling or moving your OS, applications, and data to a new drive.)

2. Can you replace the motherboard? One of the biggest problems with older PCs is not that they don't have enough processor speed or system RAM -- it's that their connector hardware and embedded controllers are antiques and any upgrade has to begin with a new motherboard. If you've got a white-box PC with a standard ATX motherboard, you're in luck. If you've got a brand-name PC with a stylish integrated motherboard-and-case design, you'll have to do more research.

   As a rule of thumb, if you open your PC case and see expansion cards seated in slots on the motherboard, your chances of a successful upgrade are fair to good. On the other hand, if you see the expansion cards plugged into a riser (an extender that plugs into the motherboard), you may face obstacles. The riser is common to NLX boards and even older LPX and mini-LPX boards, and while these are to some degree standardized, they are used mostly in mass-produced retail PCs that are generally more difficult to upgrade. Google extensively before getting your hopes up.

3. What would you consider an acceptable return on your investment of time and money? You can look at this as the difference between what you would pay for the upgrade and the cost of a similar new PC -- but be sure to compare apples to apples. If you've got a PC that can be upgraded, then it's probably going to continue to be upgradeable as time goes by, while a retail PC with the features you want may be cheaper now, but won't be as upgradeable later. And remember, money isn't everything. If you can upgrade a PC without having to reinstall the operating system or applications, you've saved considerable time and effort. Reviving an old PC has "green" benefits as well. Replacing an old PC with a new one puts one more old PC into a landfill, while upgrading it keeps those resources in use.

There is a range of possible reasons for wanting to bring an old PC back to productive life -- and some of the solutions are dead simple, while others are more challenging. Easy things like adding a drive to get more storage or filling an empty DIMM slot with additional RAM barely even qualify as upgrades. (If your PC uses DIMM memory, it's not that old, either.)

Upgrading the operating system is a frequent cause of hardware upgrades: Windows Vista, and particularly its new graphics system, makes a new set of demands on PC hardware. (For more, see Are You Ready For Vista Graphics?).

The real challenges start when you want to do something like replace your CPU. Or you can't upgrade your memory because the motherboard won't support more. Here's where you begin to think about replacing your PC's motherboard. And here's where the age of a PC shows the most.

Just about every major connector and socket on a PC has changed in the past half-dozen years. DIMM memory has replaced SIMM. PCI expansion slots have replaced ISA card connectors, and PCIe slots have replaced AGP for graphics cards. SATA has pushed out IDE for hard drives. Peripherals have all gone USB, while parallel and serial ports are quaint historical artifacts.