If you go into a folder and just look at the size shown IN the folder, then it only shows the files that aren't in subfolders. Ctrl + A and check the properties should show how big the whole thing is.

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He might be running XP - I don't think any other Windows OS has /Documents and Settings/ and all that.

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Also, to bring back some space, you could turn off System Restore, then turn it back on -- WARNING: MAKE SURE THE COMPUTER IS COMPLETEY CLEANED OF VIRUSES AND THE LIKE that way if you need to restore back, you'll have a completely clean point to restore to.

To turn them off, then on:

Right click (on desktop) My Computer > Properties > System Restore [tab] then tick the tick-box that say: "Turn off System Restore on all drives" apply it, check the volume of your C: Drive, then go back, untick it, and apply it.

With us it gets rid of about 2gb of un-needed Resore files.