I just cleaned out my HP Media Center PC m7250n tower, which consists of taking the sides off, vacuuming out the dust, airing out with compressed duster. I had to unscrew the internal fan to get all the dust out. Put everything back together. Now the unit will not turn on. The green light in the back of the tower is on, so I know the there is power to the unit. Any ideas?

1. Check by replacing the monitor if you have any standby.
2. Open the Back Cover and blow up the dust using blower and dont **** it. If possible use soft brush and remove hard dust and blow it, especially near the chipsets.
3. Try removing the memory and reinserting.
Now try closing and switch on.

Your recovery disc may be faulty try again reinstalling Windows XP

your power supply is faulty you will need to replace it. If you recently installed a new hardware it could be you don't have enough power to, either upgrade you power supply or remove the hardware.

Try to follow the link-
http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph04375&cc=us&lc=en&dlc=en&product=1118243

This would help

You need to unmute the line-in and adjust its level as necessary. Usually you can just double click on the speaker icon in the system tray and the Line In is by default muted and it is on the far right column (depending on your setup). If it is not displayed, click on Options and Properties and put a check mark by Line-in so that it will display.

Here's a step by step guide for that HP 7250 model on HP's website about how to replace, or remove/add the Primary and Secondary Hard Drives (which pictures that help).

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00639696&lc=en&dlc=en&cc=us&product=1118243&rule=29389&lang=en

If your computer did not come with a second hard drive, you can install a second hard drive that you attach to the bottom of the inside of the computer tower.

http://h10025.www1.hp.com/ewfrf/wc/fastFaqLiteDocument?lc=en&cc=us&dlc=en&docname=c00371243

That link is a video of how to take apart yor case. It is very thorough, primarily, it just shows that the side of the case is held on with a couplw of screws, looking at the tower from the front, it's the left panel that you want to remove, not the left, the right one will give you access to what yo are interested in doing.Make sure the plug is removed not just turned off. There is residual voltage that still runs into the main board from the power supply, and try to use a very light computer vacum or one of those swiffer dusters work well. If you poke around with the vacum be careful that you don't **** off the jumpers, you won't relise it untill you turn it on. You can use compressed air that is sold in computer stores or most large electronics stores like Best Buy, Don't forget to get the power supply in thr upper corner of the case, it also has fans in it that collect dust. When you re assemble it, make sure you get the panel in the groove at the bottom, and the clips at the top, and good luck

Should be screws on the back of tower to take the whole top off or screws down left side back to take the left side off.If one side can come off after screws are taken out you will slide side back to remove.
God Bless Greg

hold the start button in and check the back of the case to see if the power supply fan turns while the power button is depressed. If it turns while the button is pushed, but then stops when you let off the button, then the power supply is probably bad and needs replacing.

Cilck this Link:

This will open the driver selection page.
Select your Operating System, download all required drivers...and finally..... :))

Thanks
Iqbal