Wondering if anyone else has come across this problem - it happenned to 2 of the PC's at home within 1 day of each other, both running XP .

can't remember the exact filename but it was a .dll required for explorer ( not internet explorer but the desktop )
pc's would boot up give an error window that the file was missing and a reinstall might fix this. Clear the message and I was left with the wallpaper and nothing else, no icons, no taskbars etc.
Tried to boot in safe mode and copy the file via a usb from a clean pc, could see the file on the usb, but access denied to copy.
Had to do a repair (not recovery) and fixed the problem - bonus it kept all the settings, programs etc.