I don't have an 8400, but I don't believe there should be anything 8400-specific in track 0. The following should apply to all models. (BTW, are you saying you're using RAID or not using RAID?)
Only LBA 0 should matter, and there shouldn't be anything of consequence in LBA 1-62. I've occasionally seen garbage in 1-62, but that was not relevant to the boot process. Sometimes there will be junk in one or more middle sectors left there by other programs--often copy-protection schemes, but that wouldn't come into play until you try to use that particular program. Ghost, including Dell's PC-Restore variation, will leave junk in LBA 62, but that's non-essential (everything still works if you wipe LBA 62 clean). And the service tag is burned into the bios, not the hard disk. (Perhaps some program is including your service tag as part of their copy-protection scheme?)
IOW, the disk and Windows should still boot fine with LBA 1-62 completely zeroed out. AFAIK, Dell's boot sequence does not look for anything there. The DellUtility partition is booted by the bios, not any special boot code hidden on track 0--see www.goodells.net/dellutility for details. The bios looks for a 'DE'-type partition in the partition table, so I wonder if perhaps you haven't reconstructed your partition table correctly. Are you sure you found all partitions? What about the DellUtility and DellRestore partitions--did you erase either, or should both of those still be intact on the disk?
"But I suspect there's a restore partition on my 8400. Before the crash, XP reported a disc size of 149GB ... if we assume XP uses the unit of GB in the "binary sense", ... then 149GB (binary) is about 160BG (decimal), which is exactly the advertised size of the hard drive."
I think your assumption is correct, so it sounds like there might not have been a restore partition previously. You might try a diagnostic tool like findpart (www.partitionsupport.com) to see if it finds anything that looks like a fat32 partition near the end of the disk.