Several possibilities:

- Is your count including the subfolders, or just the files actually in the folder you are looking at? Use the properties selection of a right click on a folder to see the subfolder inclusions added in too. Otherwise, you have to add up the subfolder sizes, and then add them to their parent folder sizes.

- The registry takes up space too. But it shouldn't be that big.

- The boot file is not on the folder list.

- Is your drive partitioned? You can see only one partition at a time.

Thanks, MidiMagic, I'm getting closer to the mystery ...

You mention four points:

On the first one, I always understood that my computer told me the sum total of all the sub-folders when doing a count. So, following your post, I chose a typical sub-folder in Windows\System32 and tracked it closely.

I took C:\Windows\System32\Drivers
It reports 190 Mb
I can find only 55.5 Mb (23 Mb + 32.5 Mb)

It has 220 files scoring 23 Mb

It has 5 subfolders totalling 32.5 Mb as follows:
Subfolder 1 AVG states 30.5 Mb [4 files - exactly that]
Subfolder 2 Disdn empty - no files
Subfolder 3 Drivers. This has 5 files with exactly 1.07 Mb
Subfolder 4 Etc Has 6 files which total 238 kb, exactly that
Subfolder 5 656 kb It has 2 files. one of 656 and a file in blue lettering named: MsftWdf_user_01_00_00.Wdf which "has 0 kb"

So this is the same problem: Where is the 190 Mb? The only wild card appears to be the blue file Msft mentioned above

On the second point, I don't think the Registry has got overinflated. I use RegCure which (I hope) eliminates unwanted files.

On the third point - and different from yesterday, I now have the 4 following BLACK items on C:\
Documents & Settings
Program Files
Windows
Setuplog.exe
And then there are FIVE files in blue lower case letters: $avg8.vault$ as well as 4 files with names similar to sqmdata00.sqm

Your fourth point was: whether the drive is partitioned?
This is how the HDD is split:
C drive, for Windows XP has 19.5 Gb
E drive, for Program Files, has 48 Gb
F drive, for user files, has 43 Gb

C drive is the one giving the perpetual headache!

RussellXPD

Solution to the problem - it was a bug

This is a lengthy post but it should be helpful to whoever may catch the bug at some point in the future.

This problem was about C drive getting larger every day and threatening to put me out of business. (Windows XP Pentium 4 2006)

It was made worse because I couldn't see what was causing the expansion of the drive. I had had to move a mass of programs and user files just to allow room for expanding system files.

Daylight broke when Tosh suggested the first program that actually helped crack the problem. He suggested TreeSizeFree. I hadn't heard of it before.

When I put it to use, for the first time I saw C:\Windows\Installer. On this folder there were a large number of .msi and .msp files.
TreeSize allowed me to see this doubly hidden folder and to manage it. Specifically there were 93 .msp files, each 117,166 kb and all dated 27/7/2007 (sic). However I have only had this serious problem for 3 months.

I have a similar computer which does not have this problem. Using Treesize, I found that this machine had JUST ONE file which matched the 93 on the troubled machine.

Not really knowing what I was doing, I began to move these files onto another partition, and found out that things still worked. As a precaution, I copied the good .msp file from machine 2 in and then moved the rest.

C's free space has changed from 3% to 55%.

There may still be junk in C:\Windows\Installer but at least now the threat of being closed down (or having to reformat) has gone.

You may be interested to follow this up by googling comments by others who've experienced the problem. The first 20 posts I read (yesterday) indicated that this bug does strike hard (at random?) and that there is no patch available from Microsoft for XP Home or Pro.

Hence this longish post.

Russell