OK, thanks DavidR and bigwilly_312000.
As far as I'm aware, Win XP Pro x64 SP2 is basically equivalent to Win XP Pro x32 SP3 (which is clearly quite confusing), but I am open to correction on this.
Also, I have now tried replacing the install of Win XP Pro x64 SP2 with a fresh install of Win XP Pro x32 SP3, and I'm getting the same behaviour, ie the Avast scanner kicking in whenever I browse to a folder with an exe file, even if Avast has already scanned it. This is the first time I've used an SSD and I don't believe I have experienced this behaviour on any of the DOZENS of installs I've performed on mechanical drives (including on the same laptop), so is this behaviour specific to SSDs?
Just for clarification, I have not applied any other XP updates, ie for 64-bit: base install of XP Pro x64 w/ SP1 followed by application of SP2; for 32-bit: base install of XP Pro x32 w/ SP3. Normally I would apply a couple of XP updates after installing XP Pro x32 w/ SP3 before installing Avast, but I can't imagine their absence has caused this problem???
EDIT: In fact, the Avast appears to scan an exe file EVERY TIME I move the mouse pointer over its icon (ie without browsing away from the folder and back again). This seems VERY, VERY wrong. How can I check the activity of the File System Shield to see if this is correct?