Hi again
Sorry for the delay in replying - lots of end-of-school-year projects (PTA!).
I'd first imagined that your Presario was a laptop. If it's a desktop, it's easy enough to open up & have a look at. For a system that is experiencing no fan activity or power at all . . . there is an "out of the case" troubleshooting procedure (we use this for new builds when things won't power on, but it works for old systems just as well -- and can point to which part has failed) ---
http://forum.pcmech.com/showthread.php?t=12753 ---
Since your power supply tested OK (I'll assume the tech used a multimeter or supply tester), you know that the supply itself is OK. But you'll still have to check the paths the power needs -- the connections, the switches, and the proper setting (in general, for the US 115v, in Europe 230v). Another good early step will be to inspect the surface of the motherboard (looking for any evidence of burns or leaky capacitors).
If you can get the system to show evidence of getting power, you can move on to checking for Bios error beeps (or codes, should you get a display), and then on to re-connecting parts (reseating memory modules, checking the cpu heatsink/fan, fan power connectors, etc.), and then to replacing parts.
You can only test a motherboard that is functional enough to make it through the POST (Power On Self Test). If a motherboard can get through that, you could run system diagnostics from a bootable CD (the "Ultimate Boot CD" is a good tool for this --
http://www.ulimatebootcd.com --).
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Regardless of whether the Presario has come to the end of it's usable days, backing up the data is a good idea. One of the easiest ways to back up a hard drive is to use an external enclosure (USB 2.0 is a common connector) and copy the data over using another PC. You could then burn the data onto CDs or DVDs for longer term, and convenient, storage. There are also USB-IDE/SATA adapters you can use, instead of using an external enclosure.
Example of an inexpensive enclosure ---
http://www.newegg.com/Product/Produc...82E16817388002
Example of an inexpensive adapter ---
http://www.newegg.com/Product/Produc...82E16812186036
If you've another desktop PC around, you can also "slave" the hard drive to the other PC by attaching it via an IDE or SATA cable -- just make sure that you don't boot from the troubled drive while it's a guest in the other PC.
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Best of luck!
. . . Gary