Replaced power supply and still does not start on sr2011wm

Is you monitor cable properly connected? And as you have been inside your tower, I would suggest reseating your RAM sticks.

i would remove power and blow out the dust in the fan and then power it back up but i am gussing you have bad power supply. these are easy to replace and you can get the power supply model number and order one online
good luck

One of to thing is happening.

No or too little power is being supplied to the card.
I think this is most likely your problem the hp power supplies are bare minimum for the stock config.

A good 500-700watt PSU should be the safe zone anything less will ether not work or will slowly kill the power supply from useing too much current.

Motherboard is not compatible with a pci-e card. if there is 2 slots try the other one. Also i know it sounds a little condescending but are you 100% sure the card is 100% in the slot and that you vacuum or blew the dust out of the slot.

Well, unplugging something (I'm not quite clear on what you unplugged), especially if it supplies power to the processor usually doesn't fix things. If you mean the plug that looks like http://www.pcguide.com/byop/diagrams/figure9.jpg then it helps supply power to the motherboard, and I'm not sure what unplugging it would fix but it is more likely a motherboard issue than a processor issue if unplugging it fixed the problem. Regardless because this is an older and off the shelf computer upgrading it will be difficult due to the fact that celeron's don't have the same socket type as newer processors. If it is a motherboard issue then you will need a new motherboard and a new processor for that motherboard. Essentially the easiest and most cost effective thing would be to get a new computer.

the psu is bad.
the test is easy , one jumper wire prove PSU bad.
if its fan is dead, (and all fans )

jump pins 16 to 17 (power on pin jumped to ground 17)
connector pulled, turns it on.
see here. and why

http://www.pcdied.com/power.html#Jumper

Look on the back of the unit near the power supply for a master power switch, if none is there the it is more then likely the power supply has failed and or the motherboard itself.

There could be a number of problems such as:

Your PSU is toasted. (Power supply unit)
Your Motherboard is fried.
Your Ram is fried.
Your power button is loose from the contacts (or the wire is loose).
Your Motherboard, Your Drives, and/or Devices are unplugged in one way or another.
And/or more

Check power Supply as it might have shortened the power, Open the case and remove the big white power lead from the motherboard (this is clearly visible as all the colored leads re and yellow from the power supply - normally a P4 feed into the socket).
Unplug this, find another power supply and using the big white cable connect this to the motherboard and try powering the machine up with that.
If works - great just replace all power cables from the power supply in your machine with the ones from the working one (not forgetting to remove the old power supply from the computer and replacing it with new one).

This should work

best guess bad motherboard or bad power supply.

things to test:

1. Check that the power cord is plugged in snugly to the back of the computer and connected to a working power outlet or surge protector.

2. Check that the power cord is good. Replace if needed

3. Have the computers power supply checked. Replace if needed

4. Have the motherboard checked. Replace motherboard or get a new computer your choice.

Most common reason for this would be defective power supply, if it is less than 1 year old, may still be under warranty. If older then you can usually find replacement power supply $40+ at local computer store. Not to difficult to replace, average tech could swap it in less than 15 minutes. Can test it with one of these: http://www.antec.com/Detail.bok?no=429