It looks like the PSU is on the way out but they are easy to replace for someone who knows how to upgrade a pc. As others have said Dell can use non standards PSUs in some of their systems so it would be worth checking ebay for one and asking a friend who's handy with a screwdriver to swap them over.
If you haven't got a friend who can fit it or you can't do it yourself, get the PSU and take it to a shop and ask them to fit it. If you take it to a shop as it is they might claim the whole pc is knackered and try to get you to make expensive purchases you don't need.
It will be a 5 minute job to swap them, if that.