I have a Dell Dimension 4400 with win xp pro and 1.8 ghz pentium 4 and I recently moved it from my apartment to home and did nothing but remove an internal pci modem and insert a network card and move a pci scsi card to another slot. I had to tilt the computer over in the process and I'm thinking sometime during this or during the trip something happened. But anyway, I started getting an error saying "boot failure: system halted" and I tried everything from plugging and unplugging hard drive cables on the motherboard and on the drives and checked the power cables, and I updated the system bios. After I updated the bios the error changed to "no boot device found: system halted" which I assume is a similar error. The hard drives just aren't being recognized, as I can verify in the bios. The cd drives were being recognized though. Does anyone have any ideas as to what the problem might be?

Try removing all the pci cards. Then try to start the computer. If it starts, then shutdown and install one card at a time, until you have things either working, or you identify what card is the problem. Then check for irq conflicts or the card.

No dice on that. It wouldn't start up with all the cards removed.