PC won't boot.. What could be wrong?

The beeping means hardware failure, probably memory, hard disk or other peripheral devices. Try replacing the memory with a new one, and see if it works. If not, then you have to see go through BIOS setup and see the status of your motherboard and all the devices are noted. If all is normal, then probably a new installation of your operating system would be the solution.

I try to share my experienced here. If my pc has short signal sound, like beep beep beep beep continuesly, it's a problem with the RAM. It could be the port is damage or the RAM is damage. Take out the RAM and clean it with newspaper. Try to switch the RAM between ports, use only one RAM at one time for test, this way you can know which RAM or port that damaged. If this doesn't work, take other RAM or a new one and test it in all port. Make sure they are the same version of RAM (DDR 2000, DDR 2500, DDR 3200, etc.) If non working, means both port damage and you have to buy new motherboard. Secondly, if your pc sounding a long signal without a gap in between, that means the graphic card ic is damage, or in case you using graphic card, maybe that graphic card is damage or the port is damage. If the graphic card ic is damage, then replace a new motherboard. It's not worth it to repair the ic. In case you use graphic card, try to clean the join part (connection part with gold strip color) of that card with newspaper. Then try to put it again correctly. If still not working, try to use other working graphic card. If still not working neither, then just replace your motherboard. Last option, get other hard disk that working on other cpu and has been installed with windows, put it in your cpu to replace your hard disk. If the default cause by virus in your hard disk, this method will work. Then you put your problem hard disk on the other cpu, format it and install new windows. You also must clear the cmos by moving out the battery on motherboard and pull off the power supply cable to your computer for at least 10 seconds before doing all above step. Another thing, since you use microtower cpu, maybe you cannot find motherboard that fits with it, I suggest you buy standard tower cpu to change the micro type one. Well, I hope one of the tip will work with you.

Accordin to the troubleshooting guide - dc5750 models, page 58, 9 beeps may mean that the voltage selector on the power supply is incorrectly set, the correct value would depend on you mains voltage. It also mentions motherboard failure and or CPU failure, have you been over clocking ? You need to search for c00765931.pdf

Replace your CMOS battery with new one. Find the CMOS jumper and and clear the CMOS. Then when the computer boots, enter the BIOS and reset it to BIOS DEFAULT. Hope this will help.

Maybe the memory card of your PC is dirty or it is not working anymore. Try cleaning it with an eraser. Then, put it back again, boot your PC. if it is not working, maybe your computer had a virus because there is no other hardware problem. Search the net for other troubleshooting.