Are you SURE that the files in your recycle bin were on that drive and not another?
recycle bin on my XP sytstem shows ALL the recycled files from ALL drives until I empty the bin.
And until you actually empty the bin they are still taking up drive space.
Go to: Start>my computer>control panel>system and check the "System Restore" setting on
that drive because that also takes up space.
100Mb on a 500Gb drive?
that is ~0.02% of your total drive space. (literally 1 part in 5000)
In this universe there is "trivial" then there is the even smaller thing you are worrying about:)
As for "Used space", it'll never be zero, ever, there is always the file allocation tables to be considered
(yeah they call it by another name when you format NTFS, but....