Slasher
Experienced it on 95/98/98SE/ME and NT, usually after heavy processing or background processing.
It is residual (like a ghost/zombie). It's believed to be down to screen refreshing i.e. intensive processing means that the screen hasn't refreshed as well as it should.
Nothing to worry about, and F5 may remove them all at once.
Ping An (Be Safe)
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'I'll take control...'
'You have control.'
'Well give me it then!!'