Okay...updated, and it seems to be working!
First, had to delve into the settings a bit. I uploaded a RIP (rescue is possible) CD to the datastore along with one of our XP install disks.
Second, the bootup was impossibly fast. There's a setting to control that, and to control forcing it to go to the BIOS setup at boot, from the edit settings for the VM. Need that to change boot order to work with the CD before the hard disk.
Third, attach CD ROM image from datastore (RIP CD) to the CD drive.
Fourth, remember to "connect at startup" for the CD drive. Whoopsie.
That's how to get it to boot from another source. I booted RIP and had it run Testdisk, which did some repairs to the partition but it kept detecting that the number of heads was misset (I'd change it in the geometry menu but it just wasn't "saving" the new settings...haven't figured that one out.) Reboot, this time it got to the point where it would blue screen. Progress!
Next was a trip to Windows XP's bootable ISO and from there into the recovery console. I ran fixmbr, then fixboot, then chkdsk c: /p twice. Did a quick dir to see if files looked intact (like ntdetect and ntldr) and then exited, shut down the VM and removed the disc from the virtual drive (disconnect at powerup) and crossed fingers.
The VM booted up. YAY!
Thank you to all who offered suggestions!
Best Answer
If you're building two or more VMs from a single .ISO then I'd use the vSphere Client to copy the .ISO file to a host-accessible datastore and then point the VM's CD drives to that, it'll be much faster than linking to a remote file as it'll have direct access to the file.