omg it happens to u too
i thought it happened only to me
this thing i noticed last year August when i first installed XP. i was soooo curious to know what was really happening. honestly still dunno what is exactly happening. but now i dont care about it either. i just satisfy myself thinking that it may be defragmenting the hdd while it is idle. lol - but dunno if it is the case - who knows.
can't believe it is a common issue