- The hard drive
- The monitor (the quality of the image degrades in time as the chemicals and neon tubes inside are consumed)
- The coolers
- Power supply
- The CD ROM
- In some extreme conditions CPU, memory and main-board chipset can age sooner than expected due to overheating
Extras from the article:
While Windows XP offers two power-saving states, standby and hibernation, standby is probably better suited to a desktop computer environment. This is due to the fact that in standby the system simply goes into a low power state instead of saving the contents of RAM to the hard disk and shutting down.
Standby works by gradually putting your system into a low-power state in three stages. The first stage cuts power to the monitor and hard drives, the second level cuts power to the CPU and cache, and the third level drops down to provide only enough power to support the contents of RAM. You typically revive the computer from standby with a mouse click or a keystroke.
Configuring the system to use standby is easy. To begin, access the Control Panel and double-click the Power Options icon. If you’re using the Category view, you’ll find it on the Performance And Maintenance page.
When you see the Power Options Properties dialog box, select the Home/Office Desk option from the Power Schemes drop-down list. Now, in the Settings panel, select appropriate time intervals to gradually turn off the monitor and hard disks, and to put the system into standby during times of inactivity."