If it's a large fan shroud, simply pinch the clips on the frame one at a time until the assembly breaks loose. The clips engage on the back of the case. You might be able to remove the power supply without removing the fan shroud. It may have the same 4 screw design of most standard atx cases, requiring you to only loosen the screws on the back of the ps.
There is some really bad information in these threads.
First of all, it's become some kind of urban myth about Dell using a propriety power supply and a standard ATX power supply will "fry it".
Not true at all, not even a basis for truth, like it use to be that way on older Dells or is on newer Dells. It never was and isn't now. I believe this was started by Dell haters. It's not true at all.
Anyway, the green shroud can simply be grasped and pivoted out of the way (moving the power supply wires out of the way a little). If you want to take it completely off then it just unclips at the pivot point. It couldn't be easier.
And the green shroud can be purchased including the fan housing. I've seen them on Ebay complete for about $25. Although why you’d want to replace the shroud I don’t know.
The fan is about $13.
Now for that you do have to buy one made for Dell's 3 pin motherboard connector. If you use a fan with a standard 4 pin molex connector you'll get an error screen on every boot up telling you don’t have a fan.
Even this can be disabled in the BIOS though.
Replacing the power supply and case fan on a Dell is very easy.
There is actually some substance to that "urban myth" about Dell and power supplies. While a standard ATX power supply will not FRY a Dell, it will probably not fit in your case; there is no ATX standard for the configuration of the fan/plugs on the back of the power supply, and Dell is notorious for having their own awkward configuration.
So, when upgrading/replacing your Dell power supply, you do have to make sure that it is Dell compatible, or at least visually check (at least three times, since you will be wrong the first two) that the plugs and fan holes line up with what you have in your machine. In addition, power supplies with switches on the back will NOT fit in Dell cases.
tldr: Dell sucks; triple-check that your new power supply will fit.