Turned out that it booted up fine to the desktop. But the desktop was completely empty - no icons, no task bar, absolutely nothing. Left or right clicking the mouse had no effect. Ctrl-Alt-Del had no effect. Windows key had no effect. Windows+ (for example) E produced the expected HDD noise it is 'loaded' Explorer, but nothing on the desktop. Similarly various function keys.
Basically there was no way to get anything to appear on the desktop.
Looked as if Windows Explorer was crashing but without any means of, for example, running RegEdit (one of the possible fixes) it seemed impossible to resolve.
Tried Safe Mode - straight after displaying the module list it crashed and rebooted' Tried Debug mode - booted up and then hung. Tried Last Known Good Configuration - blank desktop.
Checked the BIOS for odd settings, all looked ok.
Eventually I went into the (HP) Recovery Mode on boot up. That set the machine back to factory settings, though didn't lose the data.
Then I tried to installer the drivers for the BT Voyager BB modem. No joy. I then realised that it had been reverted to XP SP1 and that the installer and/or drivers needed a later SP.
Today I went back with a spare router, connected that up and soon was on the internet.
Windows Update wouldn't work with SP1...
Had to manually download SP2 and SP3 and then install them.
After that Windows Update found another 125 updates....
Eventually, after removing Norton Antivirus 2004 and installing Microsoft Security Essentials I was able to run things, and access his Yahoo email.
Bit of tidying still needed but at least the system is usable.
Sorry about all that rambling but 2 questions arise.
1 What might have caused the problem in the first case?
2 What could I have done instead of going to Recovery Mode?
There may be other lessons I could have learned but at the moment I can't think what they are.
And yes, I'm working on persuading him to upgrade to a Win 7 PC and to get a router.
On that latter point, any suggestions for an inexpensive basic router (would need more than 1 LAN port), he's only about 1km straight line from the exchange so a short line.