Here are some tips for various parts of the PC.

Keyboard
Disconnect the keyboard and sop up any standing coffee. Hold it upside down over the sink and give it some good shakes. Usually you can wipe the keys with a damp cloth.

I have had success running the unplugged keyboard under water to get rid of the residue from sticky drinks. You then turn it face down on a towel and let the water drain out for a day or so.

Your keyboard may be the type that you can (gently) pry off the plastic from each key. Use a flat-bladed screwdriver or something similar - and don't force it. You may be surprised at all the dust and crud that has accumulated under the keys. This can be a good maintenance step if you find the keys not depressing and releasing as well as they once did.

Mouse
If you have a standard mouse (as opposed to an optical or other type), you will see a round plastic cover on the underside of the mouse that holds the mouse ball in. You can usually turn the cover a little in the direction of the arrows and take the mouse ball out. Then blow out the crud that has accumulated, clean the ball if necessary and replace.

Mouse pads are cheap so it's normally best to just get a new one if something spills on it.

Case
The case is probably made of metal or plastic and can be cleaned accordingly.

Monitor
As cautioned above, don't spray directly on the screen - spray the cloth first and then wipe it. If you have a laptop or special LCD screen as opposed to the standard glass screens use a special cleaner designed for them. You can get little packets (like they give out at rib joints) of screen cleaner designed for cleaning your monitor. Occasional cleaning will remove the dust that is attracted to the monitor.

Inside the PC
If you are comfortable with taking the cover off your PC, you can do some preventive maintenance occasionally. Use a can of compressed air to blow out the dust that accumulates inside the PC. This dust gets sucked in and contributes to overheating of the chips and could cause problems.

I actually had one client whose computer fan stopped because of all the junk inside and the system overheated and crashed. It's always a good idea to keep the side of the system with the cooling fan at least several inches away from a wall and off the carpet.

Printer
There are cleaning kits for ink jet and laser printers that include a sheet that you can send through to clean the heads. Many newer ink jet printers include routines to clear the ink jets. Run them as recommended by the manufacturer.

You can clean the heads of your dot matrix printer with a cotton swap and a little solution after removing the ribbon.

Be careful. Printers can have some very small and fragile parts that can be damaged easily. Compressed air is a safe way to start blowing out the junk that accumulates.

If you really doused the keyboard don't despair. New keyboards cost from $10 for basic models to $80 or so for feature laden units. Consider it a fairly inexpensive lesson learned.