Don't throw out that computer or replace any components just yet.
After much trial-and-error, I finally figured out the solution for this common issue with certain HP Pavillion models. It turns out to be a design flaw with the front-panel memory card reader. Somehow the card reader's firmware gets scrambled, and it prevents the computer from booting. You can test this on your machine by temporarily disconnecting the card reader cable at the motherboard header. Voila - I'll bet you $1 that your computer will now boot!
The solution is buried deep inside
this HP tech support document. The card reader firmware needs to be reset.
Here\'s how to fix it once and for all:
1) With the computer power unplugged (duh), open up the case & disconnect the card reader cable at the card reader itself - NOT the end that connects to the motherboard header. Make sure the cable end is not touching any metal components before proceeding to step 2 (another duh).
2) With the cable still disconnected, fire up the computer. The tech support doc says to let it boot into Windows and stay there for about 5 minutes.
3) Shut the computer down and reconnect the cable to the card reader.
4) Turn your computer back on - your memory card reader will work and, most importantly, the computer will no longer hang at POST!
I suspect that when you fire up the computer without the cable connected, the card reader is designed to reset its firmware by grounding out through the case - sort of like a CMOS Reset jumper on a motherboard.
I hope this solution is helpful to others.
best,
JohnB