Computer help forum: Replaced power supply unit now computer wont boot up

Im hoping someone smarter than me can lend some advice. After replacing my power supply I cant get the computer to boot up.

Some background:
Computer is a HP Pavilion 405N running Windows XP home edition purchased new 2-2003. System has worked nearly flawlessly until a power outage hit recently when the computer was powered up.

After the main power came back on, computer was ticking with power light flashing (both on front panel and LED at PSU) and would not power up; I found and used HPs troubleshooting page on this, and ultimately determined the PSU was faulty and replaced it. Computer now powers up (power light on, CPU fan ((?) fan on heat sink attached to motherboard) operates however the monitor wont wake from sleep mode and the familiar sounds of a hard drive or other functions working is not there. Monitor power indicator is orange unless I turn it off then on, it comes on in blue (normal on-wake color) quickly gives me the red green blue power saving screen, then goes to black and power switch color changes to orange (normal sleep color). Original PSU was a Bestec 250W which I replaced with a Silencer 310W from PC Power & Cooling.

My understanding, rudimentary at best, is that the front power on switch is connected to the power supply through either the CPU or the motherboard. Im guessing that since the power switch functions as it should (powers up and down correctly) at least some parts of the system are ok.

So, where do I go from here? I have found and am using Foner Books troubleshooting guides, but since the sum total of my knowledge is what I have found online Im hoping someone can steer me in the right direction.

Thank you for any help you can lend.