Ok, so it's not likely to be the caps then is what I can gather from peoples responses?
I don't know if caps go open or short circuit when blown.
Sceadwian, what I am talking about is the Dell I am talking about in particular does not use a standard motherboard, it uses a proprietary one designed by Foxconn as part of their G33 range specifically for Dell. The G33M02, the ATX connectors are all standard etc, but the motherboard isn't, I could replace it with another similar micro ATX but would have to start cutting the case as the Dell case wont accept any other board supplied backplanes, plus the O/S would be defunct as the O/S certifies through the BIOS, to check it is a genuine Dell BIOS. All of which would be less painful to either:
1) Fix it! Which is why I am asking on here.
2) Find a replacement, not easy since they are obsolete, got a few companies to try.
3) Replace the case / base unit and save some of the components, but I may as well get a new unit for the sake of £180, plus £50 for a Windows License.
If someone had said "yes, I have had this before, just replace the 3 or 4 1500uF capacitors and it will work fine" then I would have attempted to fix it myself, as it seems they are the most popular component in the failure of board power supplies. I'm not going to "hack" it to try and make it work, if it needs a new board then it needs a new board, I may well get a new board and keep the old one to experiment with fixing it.
I appreciate all of the responses, I do not have the time to fault find that deeply, so as mentioned I may put it to one side for a later project.
I will say that all of the Dell machines I have owned, which have been a few, they all use proprietary motherboards (usually popular models of existing boards with a slight change), some other mobo's have the same backplane, but this particular one I haven't found one that has the same backplane, other components such as PSU, Drives, CPU, Memory, Graphics boards are all standardised components.