Until a few weeks ago, my Dell Dimension 8250 would come to life within seconds of pressing the power button (with a green power-on LED). Over the last couple of weeks it has taken progressively longer each day until today when it took several minutes to power-up. After pressing the power button nothing happens for many seconds to a few minutes, then I see the LED in the power button blinking yellow for about a minute, then finally the LED turns green and power-up occurs. Any ideas as to what is causing this?

I believe that is the cause of your power supply, this normally occur in recent computer with "new manufactured power supply" i think it's the maker design flaw but anyway it causes a "charging up effect" then finally powering up due to some reason like switching off the main power switch. If switching on the main switch at all times solved that "charging effect" then my statement is correct. NOTE the power supply is still working just that it has this weird charging up effect - perhaps a design flaw then again i'm no engineer A new helpful user

I would look at 3 things. 1 power supply (newegg has some inexpencive testers). 2 is ram d/l memtest and run it. 3 it could a hd failing. Newer systems use a "smart" system that will attempt to move files if it detects bad sectors before the sys boots. Run a smart test on the hd on the road again!!