PC fan spins after power off - HELP

I built my PC about 3 years ago. CPU Core 2 Duo 6600 @ 2.4GHz, Video – 7950 GTKO; MB – Intel DG965WH, 2Gb ram, XP Pro.
Recently I noticed that PSU fans as well as the chassis fan and CPU fan would continue to impulsion run well after the machine was powered down.
The only latest add-on was an external Seagate Free Agent Pro – 1TB and it was few month ago.
I fully cleaned the system of dust but no change. Yesterday after shutting the machine down I found it would not start. No beeping motherboard or hard drive spinning up, etc... After unplugging and cleaning again (for nothing) I was able to get power up only to discover “Unmountable Boot Volume”. Went on the net, found MS “support.microsoft.com/kb/555302”, used original CD to repair system – as result PC boot up. Shut it down. Went to work and now I have a same picture. I got only spinning impulsion of the fans and no way to turn the PC on. If I would turn power off on the surge protector, spinning would stop, but it would continue with the power back on. At some point I was able to get to the BIOS setting and check voltage; I got +11.88, 5.22, 3.32, 1.2.
Thank for the help