There's no need to re-invent sliced bread, so this post will just point you to 2 other articles that thoroughly explain how to clean your PC and laptop.
The main problem with dirty computers is that they are unable to dissipate heat as well as they should and they slow down to protect themselves (or they don't slow down and destroy themselves by overheating).
Of course a dirty machine is also a health hazard.
In humid regions the dust and dirt can collect moisture and lead to corrosion and electrical problems.
If cleaning seems too hard, give me a call.
You should note that, depending on the age and state of the machine, that moving it and cleaning it may in fact kill it.
You always need to clean carefully, but be extra careful with older machines.
Backup your data before you start.
Happy Wabbit hunting.