Downloaded holiday photos to My Pictures as normal(5 years).
Try to view thumbnails(again as normal)" windows explorer has detected a problem".
Whole system freezes and has to be rebooted.
All previous downloads are fine as is this batch on Picasa & HP Image manager.
Why do you even use those programs ? Windows Explorer has a thumbnail mode too, but it sounds to me like too many photos has been stored in one folder.
Create some kind of division between photos, say events, dates, month or year.
Create folders for each one of those divisions, move photos in DETAIL mode, to the specific folders. View thumbnails again by folder.
You can try delete cash on Picasa and HP photo managers.
The real test is to view the folder from windows explorer alone, nothing else open.
If that error pops up again you either have a virus or a corrupt system file.