Can you access the BIOS Setup by pressing F2 or boot the system to a bootable CD or floppy?  Do the diagnostic light codes on the back of the system show any kind of error code?

If you can access the BIOS Setup then I doubt the problem is BIOS related so therefore a BIOS upgrade won't help.  To help rule out any conflicts between the BIOS and the hardware information stored in the NVRAM try clearing the NVRAM.  If your machine has been in use for quite a while then I would suggest replacing the motherboard battery as well.  The 3-Volt, CR2032, coin cell will cost around $3 and is available at any computer store, Radio Shack and most discount stores.  While the battery is being changed you can clear the NVRAM by following this procedure....

 

With the machine unplugged from the wall remove the battery from the motherboard.  Then with the machine still unplugged press the On button for several seconds to dissipate any remaining electrical charge on the motherboard.  Then re-install the new battery, plug the machine back into the wall and see if it will boot normally.  You can click here for information on how to remove the battery from another model's User Guide.

 

If the new battery and clearing of the NVRAM doesn't resolve the problem then try booting the system to a bootable floppy disk or CD?  Booting to either one of these items will confirm the system is booting up properly.  If it is then I would suspect the hard drive or the operating system installed on it as being at fault.

I am assuming then that the system was successful in booting up with another hard drive installed??  Not quite sure what you mean.  Did you install another hard drive and then install Windows or did you reformat the existing hard drive and install Windows?  I am assuming that by the term "all my stuff to this new setup" that you put a new hard drive in and wish to get files and information from the old drive.  If so then connect the old drive as a slave to your new hard drive and then see if you can copy what you need using Windows Explorer.

If you installed Windows to your original hard drive and it isn't working then consider reformatting the drive and re-installing Windows.  Possibly something went wrong with the previous installation.   Click here for Denny Denham's web site and the information he has put together on installing Windows XP on a Dell system. 

 

If the re-installation doesn't work following Denny's recommendations then consider running the Dell Diagnostics on the hard drive to check it's condition.  Possibly your hard drive is going bad.