AF QuickScan accesses other disks.

ASUS M4A785-M, AMD Phenom II X3 2.3Ghz, 3GB Crucial DDR2, Rocketfish 500w P/S,
C: Seagate Enterprise_2 1 TB (931 GB), H: Mushkin Chronos SSD 120 GB (111.6 GB)

I have no more installations available on my Windows XP Product Key, so I could
not do a fresh install of XP on the SSD. I used Norton Ghost to ghost the contents
of the C: drive to H:, the SSD. Using Regedit, I changed the few instances of “C:”
to “H:”, and it seems to be working passably fair - hiccups, but no complete chokes.

I’ve been running Windows XP SP3 for a couple of years on this rig. But by using my
new SSD (partition H:) for system and programs, I want to have Windows power
down disks after 1 minute to reduce the bearing wear on the HDD, in order to make
it last a while. (In Windows XP SP3, Power turns off power to all drives or none, but
not individual drives.) A 1 minute interval allows the HDD to be powered down most
of the time, without impeding the SSD, which should power up almost instantaneously.
But something accesses the HDD intermittently, which causes some long hangs while
waiting for the HDD to spin up. I may just unplug the HDD temporarily, but first, I
want to know what is accessing it.

I think it is probably the Avast! Free anti-virus program running in the background.
When I run a Quick Scan, AF scans the correct folders on the H: (boot/system) drive.
But Quick Scan uses the last 15 seconds or so to scan quite a number of folders on
the C: drive. Because the Windows installation on H: shows no instances of “C:” in
the Registry, I think that scanning C: may be a hard-coded, automatic function of AF.

Is that correct? If so, AF may not work for me when using the SSD as boot/system.
In that case, I probably should bite the bullet and “upgrade” to Windows 7 (or 8?).

I have no more installations available on my Windows XP Product Key
1 license = 1 install, unless you have a corporate license for multiple systems.
I want to have Windows power down disks after 1 minute to reduce the bearing wear on the HDD
Don't worry about that. A hd bearing will normally last longer than you will become. Stopping/starting will shorten the lifespan more than keeping it running.
I used Norton Ghost to ghost the contents of the C: drive to H:, the SSD. Using Regedit, I changed the few instances of "C:" to "H:", and it seems to be working passably fair - hiccups, but no complete chokes.
Norton ghost? I think you mean Symantec since Norton has been taken over by Symantec many years ago. C: is normally the main boot drive, so if you "ghost" it to H (the SSD), you should set H: (the ssd) as boot device in the bios. There is no need to change anything in the registry.
But something accesses the HDD intermittently, which causes some long hangs while waiting for the HDD to spin up.
Ofcourse something does. It is the bios. You haven't changed the settings in there to change the boot device to H: (the ssd) and besides that, it is very likely your bios is set to automaticly search/check for devices.

Best thing to do is start from scratch. Change bios settings so that the ssd is C: (main boot device) and do a clean install of windows. That will make sure that there will be no rubbish on the drive/in the registry which can/will cause slowdowns/problems

The things/problems you see have nothing to do with avast! but with the way you did things.