Where's all my free space gone?

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ok,a 40 gig drive isn't massive but i cant see how it's full already.noticed recently that pc was slowing,went to defrag only to find less than 1% free space.did the usual scans to check for malware and viruses to find no infection.purchased new external drive and backed up c drive only to find that backing up a 40 gig c drive took up 46 gig on my new e drive. Funny,i thought. any ideas. thanks for reading.