I'm with Keef on this - if I take a machine to a local checker-outer, I'll probably get some spotty oick - is that how you spell it? - who just plugs it in and says "It's working OK". If it doesn't work, he'll (or she'll) just replace bits that have gone brown at the edges until it does work again.
I think I've built enough of these PC things to spot things that are amiss and can do the same diagnostics as aforementioned oick without paying through the nose for his/her expertise. If it is dying slowly, then so be it - I'll fix it when it breaks next time. (Oh, and I do do regular back ups of my data, just in case).
P.S. - Power supplies do seem to be a regular repair/replacement on both my machine and others for whom I seem to be a freebie consultant!