Just like a car, your computer will benefit from the regular attentions of a computer expert.  If you wait until it breaks down, then the consequences may be much more serious .

Hard Drive Errors
Computer disks need to be scanned for hard drive errors and corrupted sectors need to be found and fixed. 

Hard Drive Defragmentation
Hard disks also need to be defragmented on a regular basis or they become slower and slower. Many programs spend the majority of their time waiting for the hard disk to read or write their data. If this is further slowed down then the whole system is slowed down.

Viruses
Viruses are malicious software designed to spoil your system in some way. A totally unprotected system exposed to the internet will quickly become unuseable.

Registry
The registry is at the heart of windows, but unfortunately can be cluttered with broken links and empty references that slow down its use.

Dust
The fans on your computer are there to keep everything working at the right temperature. Unfortunately these can also suck up dust. This is a particular problem when the system units are placed on carpeted floors.

This dust can cause overheating of the electric parts. When too much dust builds up inside your PC, the computer components get too hot and will burn out or not run as fast as they should.