Getting on my fucking tits now
Googled and found info stating it's an XP bug that mostly relates to AVI's.... One fix is to cut and paste it to the desktop, then delete. Did that and it worked on one file, but the other isn't working. Then tried three recommended "kill" programmes (Dr Delete, Sure Delete and MoveOnBoot - all of which schedule the file for deletion when windows boots, to avoid Windows trying to "catalogue" the avi) - none worked
FUCKS SAKE