As a recap, I've already tried FIXBOOT, FIXMBR, things having to do with the NTLDR, and Windows repair installation. None of these things work. The situation is that I have a bootable hard drive, and then after a random amount of times, it simply won't reboot. Yes, I've checked for viruses and malware. It could of course be something that none of the virus-checking programs catches.
Another bit of information I found while trying to find something about this problem that I hadn't seen before — most of the advice on the internet involves variations of stuff I've already tried — was a tidbit that the Dell D600 laptop's BIOS (latest revision A16, which I do have) doesn't handle hard drives >137GB very well. Some of the Dell documentation says that while the BIOS still will tell you that you only have a 137GB drive, it can actually address everything above that with WinXP SP3 or later operating systems, but other things online suggest that once something on your computer writes anything above 137GB, the drive will get confused and refuse to boot.
I have four 320 GB hard drives, which amounts to quadruple redundancy, and I've needed it with this stupid no-boot bug. I'd hate to not be able to use more than half my drive capacity. So I decided on a drastic system rebuild (again) with a difference this time. The original drive mostly as shipped was only 40GB, and I keep it under lock and key in a fire safe. I cloned it into a 60 GB partition on one of the drives. I then put the original back in the safe, put the just-cloned drive into my computer, and it boots just fine. I then added two more partitions to the drive, roughly half and half, and each less than 137GB.
Naturally, the first time I hooked this machine to the internet, it screamed for updates. First I downloaded and installed the latest AVG anti-virus, and while that was going on, Windows downloaded some 51 updates, plus a handful of others that installed after the necessary reboots from the first batch.
I don't share this laptop with anyone, so I reckon it's okay for me to sling my stuff all over it. It also seems like a good idea to keep documents separate from programs when possible. Furthermore, it seems like the largest single consumer of my document space is videos and pictures, plus some music. So I moved My Documents from its default location on the C drive to the E drive (D is the CD/DVD). I furthermore moved My Music/My Pictures/My Video to the F drive partition. Having done that, I restored the documents from the most-recently-used hard drive (that also took a couple of hours).
Now I'm going through the laborious process of re-installing software. But being paranoid about this, I'm going slowly, and after the next re-installation (the drive-cloning software), I'm going to make a complete clone of this new 3-drives-in-1-box before installing any more software. And I'm going to reboot after installing each new program, and probably re-clone several more times along the way. If it really is a software incompatibility with one of the programs, I'd like to try and isolate it!