I guess the obvious question is why you want to do this? Low end stuff tends not to need much cooling, and if you have the money to buy fans then you probably have enough money to just buy a better case, a basic one is pretty cheap and often comes with fans fitted.
There is also the potential problem that if you are running a stock dell PSU/motherboard they are likely cut down to essentials, and you might not be able to run fans without adaptors etc.
The pictures aren't very clear at all, but I'm assuming your case is roughly like thishttp://www.lulusoso.com/upload/20120309/DELL_OptiPlex_360MT_computer_case.jpg
with a kind of mesh front.
At a glance it looks like there is plenty of space inside to mount a fan, but both tape and (most) glues have issues with heat/vibration. In layman's terms, warm them up and wobble them around a bit, and they just fall off.
My advice - don't bother.
If you aren't having issues with overheating, then leave it be, it's more hassle for little benefit.
If you are having issues with overheating, try and find the cause. In an older case it's likely to be a build up of dust somewhere.
If you just want a better cooling solution for better components, buy a better case.