Incorrect disk space

Hello,

Iv'e installed XP pro on my new computer.
after installing all the software i need on the computer i checked how much disk space i'm using before creating an image, and then i encountered a strange thing.
When i measure th used disk space using ALT+ENTER from the root directory all the file amounts to about 9GB (all visible and all accounted), but when i check the disk properties it reports over 20GB used.
When i created the image, the file weighed over 20GB also even thought i don't have 20GB of files.
I know for sure that this is very unlikely that i'm actually using 20GB and that ALT+ENTER reports a wrong size because on my other computer. which is my primary computer I have a lot more software (some of which is pretty heavy-weight) installed and a lot more junk collected along the years, and it all amounts to less then 17GB. by the way, the two systems has same OS version, the new one is even a clean SP3 installation without too many patches, so it is suppose to be even thinner.

I ran chkdsk but no errors were found. what can I do ?

thanks in advance !

of course i did.

all hidden and system files.

yes, i' not a novice user, no system or hidden files excluded.

i believed maybe it was a file system error but scanning the disks didn't help finding any.

the problem is that i want to make a clean backup image on a removable device after installing the computer, but with this problem it takes more than two times the actual size.

furthermore, there is a problem and i want to fix it.

Those were hidden, weren't they?