Cannot boot windows xp, hard drive failure???

I have Dimension E521.

I ran into this trouble yesterday afternoon after my windows freezed and I shut it down. Then I tried to boot it but after the Dell logo showed up and before the windows logo should show up, I got a black screen, no cursor blinking.

Here is the thing I did and the syptom I got:

1,F2 and F12 key still work, I can access the BIOS setup and the Num LED is on

2, reboot the computer and press F8, no thing show up, I cannot even enter the safe mode

3, I tried to boot from a reinstallation CD. After a prompt saying that press anykey to boot from CD and I pressed enter, there was a line quickly showing up and disappeared, then I got black screen again. So I cannot boot from a CD as well.

4, I opened the case to take a look. But since I am not an expert in hardware, I can just tell that the fan of the CPU is always working, the driver, might be working at the begining, since I heard the noise, stop working after couple seconds. Same as the CD-ROM.

5, I called Dell tech support and they told me my hard drive is broken and I need to buy a new one(the warranty expired). Then I turned back to my computer and reboot it. Used the diganositc utilities after I pressed F12, everything looked fine sinced all the tests were passed. So I am really not sure if it's caused by hard drive and I'm afraid even I buy a new hard drive, it won't work.

So I'm looking for help to see if anyone can give me any advise for my current situation. Thanks.