Mouse stank, edited excerpts from http://www.doc.ic.ac.uk/~jab00/mouse/

3 Stench Ridden Days...

Yes thats right, for almost 3 days I couldn't find what was causing this awful smell. All of my house mates and I were convinced that there was a rotting mouse either under the floor boards or in the wall. Well I say all my house mates except for James, who suggested the smell was comming from my computer. I laughed and called him a fool, claiming that the computer surely wouldn't work with a dead mouse in it...

I am sorry James, you were right, I was so wrong.

FAQ

Here are answers to some frequently asked questions:
Despite the fact that some of the mouse's flesh burnt onto the mother board, and had poop around one of the capacitors, the computer did and still does work.
The mouse has now been removed both from the computer and the house.
Yes, I do now realise it was a mistake to lose that ATX back plate. I recommend nobody else does this.
The mouse was a lot harder to find than one might think, even though my PC never has the lid on. It was very difficult to pinpoint the exact location of the smell over those 3 days (I suspect this was due to the PSU fan blowing directly across the mouse, making the smell appear to come from the other side of the room).