Here's what I did when that happened to my computer:
1. Make sure the computer is on. This will insure that all exposed surfaces, such as fan blades, etc, are covered. If your computer turns off when the case is off, then you may need to tape down the interruption switch.
2. Try to work the cables out as much as possible while retaining the connections.
3. Find a shower with a removable spray head, or perhaps a kitchen sink will work, although a back stop is key to reducing overspray.
4. Thoroughly spray down the entire inside, taking care to direct the spray under the edges of the case and inside all of the components. While hard drives are sealed, you should still try to get the exposed contacts to prevent dust from creating spark bridges. Shampoo works best; Johnson's may be ok depending on the level of dirt, but I suspect that it's too gentle for most computers that have sat for a while.
5. Don't forget the conditioner, as this will make the cables more compliant and easier to route back into their respective channels when reassembling.
6. At this point I decided to get a new computer regardless because the scent of the cat spray was difficult to get out, even after repeated applications of Sebastien Shine With Pro Nutrient Fruit Essence No. 5.
