The freezing persisted in safe mode (which is weird).
However, yesterday ATI released a new version of their Radeon drivers (at
least it became available yesterday via their UK website).
With this in place, the freezing has vanished.
So, for ATI Radeon series cards, the moral seems to be:
1. The MS-certified drivers (as originally used and regularly updated from
the MS site) are defective.
2. The latest non-MS-Certified drivers are OK.
The MS generic VGA driver (the safe mode one) has problems driving Radeon
Thanks for all who took the trouble to respond.