Dell Inspiron 518 Can't boot Windows 7 32-bit Full Install DVD

Dell Inspiron 518 was running Windows Vista HP 32-bit crashed running updates. Could never get back into Vista OS despite attempts at every known combination - Actually installed a "test" upgrade version of Windows 7 HP 32-bit ... it seemed to work okay. Also attempted to delete the "Windows.old" folder later. It failed on a file. - - Windows 7 too crashed and burned eventually after applying a series of updates and rebooting a bunch of times. Installed the latest 1.08 Dell bios (up from 1.05). Worked for a boot or two and eventually couldn't get back to the Windows 7 32-bit OS - even in safe mode.

Purchased the full version and a new replacement hard drive (because some of the errors I got seemed to single out the hard drive too). Sometimes when I boot the bios doesn't even see the hard drive! Tried booting the new Windows 7 HP Full Version Install DVD via the DVD Player ... it consistently hangs up on the colorful logo splash "Starting Windows" for about 2 minutes and eventually goes into a black screen with a white mouse pointer. I seem to recall it looked okay on "Windows compatibility" tests before any of this went down. I exhausted all resources with my last Microsoft tech call. I figure it's a waste of time installing a new hard drive in the desktop if it can't boot the new OS via DVD. Could it be memory? - I seem to recall it hurdles these tests (4 GB of RAM). Could it be the BIOS? or the Mommy board?

Does anyone out there have any "brilliant" suggestions? Is there some great downloadable, bootable diag software out there or something I could purchase?
I have access to plenty of web resources - -on my current (working) desktop - - also a Dell Inspiron 518 with 6GB of RAM running Windows 7 64-bit HP