PC turns on but only fan spins, PC doesn't boot

SOURCE: acer

Unfortunately you are experiencing a hardware failure and will need to have a technician diagnose and replace the faulty hardware.

SOURCE: my Presario SR1750NX won't boot up

How long are you pressing the power button in? Try pressing and holding the power button for 5 -10 seconds. The computer SHOULD shut off then. What version of Windows are you running? Based on what you said, there is a slight possiblity that your video card is no longer functioning. Let me know what happens.

SOURCE: my Presario SR1750NX won't boot up

hold the power button for 10 seconds it turn off ur cpu and then start the cpu and hold F5 key u get windows boot menu select safemode try to login after login check in the controlpannel poweroptions chek in advance option see the menu down wheather it is in standby then change it to shutdown and apply the setting. and check it out it may work r else please try to repair the os.

SOURCE: pc wont boot up, instead a red light flickers.

Flickering LED means power supply is overloaded with something. It might be inside of the supply, but sometimes it is something else. Try that:
1. Disconnect everything from the motherboard except power supply and power button;
2. Disconnect all power connectors from all drives;
2. Remove any cards from the motherboard so all you have is power supply and motherboard with CPU and RAM, nothing else. You can keep connected the monitor and keyboard;
3. Press Power button. Will motherboard turn on normally?

If no - check electrolytic capacitors especially around the CPU, if some of them are "popped" replace them.
If yes - connect drives and cards one per time to localize the problem.

SOURCE: added new ram to compaq

Try taking the newly installed RAM out and re-seating it. If the persists you might have a bad block on the memory chip