Try using Windows Explorer to investigate.
Right click Start
Click Explore and a new window appears with 2 panes.

Look on the left side for C: and click the plus sign beside it causing the sub folders to drop down.
Click on the Windows folder to view the contents in the right pane.

What anti virus and anti malware programs are you running. Have you kept these updated? Run both with a full scan to be sure you are clean.

there are some that will mess with system restore points, I would run an updated malwarebytes scan and also an updated superantispyware scan, and an online eset scan. To make sure you are clean then you can determine what else could be causing your problem.
SUPERAntiSpyware

Malwarebytes' Anti-Malware (Win) - Detecting and Removing Malware

Online Scanner
on all of these choose the free version not the trial versions.

post back with results

I tried using "My Computer" to look at the subfolders in Windows, but couldn't find an obviously large one

Try using the tool below too, see link,

Here is a link that might be useful: windirsta

ravencajun:
I ran the virus/malware scanners
Nothing found except 38 cookies (deleted).

It's really doubtful this is a virus , one of the folders in the windows folder has ballooned for some reason. See if you can find it, it has to be there, the Windows folder in Xp should be around 2- 4 GB. It's possible the system restore folder has grown or some other folder like that. That tool I posted above should help in locating it

Run this too,

Clean out your temporary internet files and temp files.

Download TFC by OldTimer From here click Here
to your desktop.

Double-click TFC.exe to run it.

Note: If you are running on Vista, right-click on the file and choose Run As Administrator

TFC will close all programswhen run, so make sure you have saved all your work before you begin

* Click the Start button to begin the cleaning process.
* Depending on how often you clean temp files, execution time should be anywhere from a few seconds to a minute or two.
* Please let TFC run uninterrupted until it is finished.

Once TFC is finished it should restart your computer. If it does not, please manually restart the computer yourself to ensure a complete cleaning.

zep516:

I ran the stat tool. No particular file, but it found NUMEROUS (hundreds) of .msp files. WHat are these, and can I delete them?

I'll run the cleaner you mentioned tomorrow. Thanks.

Those msp files are uninstallation files for various Windows updates, hotfixes, etc.
You can safely delete at least some of them.
Sort them by date, and delete all older then, say, 3 months.

I found one file, mpsetupxp 9,900 kbytes!

This is related to Windows media player. Why such a large file? Can I remove it? It sounds like this is half of the 20GB Windows folder!

Oops. I misread, 9,900kb isn't what I'm looking for. Wishful thinking :(

Do you see a folder called installer or something like that?

zep516:

Yes, I have such a folder, installer

Other than the contents of temporary folders, I'd advise against trying to delete anything in the Windows folder. You risk bricking your machine.

A PC with such a small drive has got to be quite old. Try moving data files off or simply using an external drive if you need more room. It's a cheap solution if you're not ready to replace that antique.

snidely:

I have two of these "antiques" , and they are virtually identical machines. Yes they are five years old. Excuse me.

My other PC has a 40GB drive with only 10 GB in use, and it runs quite well, thank you. I don't plan on replacing it any time soon.

I'm just looking for an explanation as to why the HDD is getting populated. Buying a separate drive does nothing to solve the cause of the problem.

@tom418:

I would suggest that you follow the recommendations from zep516, and you will be just fine.

If computer works ok otherwise, I would open my computer, right click the main drive, C? ... choose properties. Use Disk Cleanup there. also System cleanup is there and will remove all but your last Restore point but clean up some other files too.

Restore points often will screw up when the master boot record or partition tables get changed or messed up, which happens with use something other than windows like PartionMagic to make or resize a partition. Or hard drive failing.
After saying/thinking, I would also run a diskcheck from the same C drive properties/system tools. Maybe do that first.

This is a free program I have have used in the past to find out what and where the large files are it will show you right away. Similar to the one zep linked to.
xinorbis.com