Amgpac has a PC that hasn't been used in three years. He asked the Answer Line forum about any precautions he should take before booting it up.
The most important thing is to make sure it's clean on the inside. If there's dust blocking any of the air passageways, the PC might overheat and damage itself.
The best tool for cleaning a PC's innards is a can of compressed air, which you can buy at any computer store for less than $10. If your old PC is a desktop, unplug it, open it up, and use the can to blow away the dust.
Laptops are trickier. I generally recommend against opening these up unless you really know what you're doing. But you can remove a fair amount of dust by blowing air through the laptop's vents.
I don't think you need to take any more precautions before booting, but don't expect to be productive on the old PC right away. Windows and a whole lot of programs are going to insist on getting updated. Your antivirus and other security programs are your top priority--and they'll let you know it. You'll likely have to install whole new versions of them. Then you'll have to update Windows as well.
Read the original forum discussion for other people's thoughts.