Dell 8300 - How do I Open my CPU (Embarrassing, I know!)

My computer was in a room in which people smoked. I was cleaning the other day and saw a dust bunny hanging out from a amall open area on the CPU.

I haven't opened my CPU since buying it 3 yrs ago - if dust bunnies are hanging out of it, I'd hate to think of what's INSIDE of my 'puter.

So.... I'd like to open the CPU and clean it out. However, I have no idea how to open it!

I'd really appreciate some assistance.


Thanks!