Well, the Win 7 installation job passed off successfully but wasn't as straightforward as I'd hope - for four reasons.
First, the PC's IDE (PATA) optical drive wasn't working. It turned out that the data cable which runs from the drive to the motherboard has been put in back-to-front and at an angle, completely destroying the socket on the motherboard (some pins were pushed right out the back, others were flattened or massively bent!).
There was only one such socket on the motherboard but, fortunately, there were three free SATA ports and I had a spare SATA DVD burner sitting on the shelf waiting for just such an occasion.
Trouble is, the cabling of the PC was all tied up underneath the motherboard - so finding the right power cable for the SATA DVD burner was a bit of a struggle (yes, really, it was - I ended up just cutting away all the cable ties and redoing them after I'd finished).
Third, although the data did get copied from the drive that was to be used for the operating system, there were problems getting it back on from the external USB hard disk after the installation of Windows 7 - some stuff just wouldn't copy over.
Turns out that a few (happily unimportant) files had got corrupted and were preventing entire folders from being copied; so I had to track them down and only copy the stuff that wasn't corrupted. Oh, and there was about 150GB of data, so the transfer took a bit of time.
Fourth - I had no issues with Windows 7 except for (hawk, spit) a relatively expensive Creative Labs' sound card, for which (par for the course), the company hasn't produced any proper Windows 7 drivers. As a result, the card wasn't recognised by Windows 7 and the system had no sound output.
To ensure that there was some sound, though, I disabled the card and turned back on the sound that's built into the motherboard.
However, the client did later manage to locate some beta drivers, which he believes will enable him to get the soundcard working properly (after turning off the onboard sound, of course).