Sounds like a failed HD...if the HD with the boot sector is corrupt then it will not load an OS. The workaround is to throw in a floppy disk or CD with a boot disk on it (like a linux distro) and change your bios settings to hit the CD/floppy as a higher boot priority. That way you can boot from the floppy/cd instead of the bad HD. You usually need a total reformat, a new HD or new PSU, speaking from experience.

Edit: sorry initially misread. I would recommend a floppy/CD boot disk regardless. If you can't get that far then it is the power supply or motherboard, both of which Dell skimp on quality. Sometimes you can short the BIOS chip or do a similar action via taking out the motherboard battery (button type). Other than that you're looking at a new motherboard (probably new PC at that age)

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth