One question I often hear from small office/home office (SOHO) users is "What happened to all my hard disk space?" The less technically savvy (and occasionally even the technically acute) are unaware of all the factors that reduce available disk space on a Windows computer. Lets look at the most common culprits.

Culprit number one is marketing hype. OSs measure storage in multiples of 1024, so 1GB is equal to 1024MB. But when vendors sell hard disks, they use 1GB as equal to 1000MB, so when you look at a 200GB hard disk, which has a capacity of 200,047,001,600 bytes, it actually has only 186GB of available storage. IT folks typically understand this discrepancy, but hard disks with 250GB of storage are missing a lot of storage compared with what people often think they're buying.

The types of files deleted by the Disk Cleanup Wizard (e.g., temporary files that an application creates, Recycle Bin files) can also reduce available disk space. It's amazing how much storage can be taken up by temporary files, especially if the applications don't clean up properly after themselves. And the problem is exacerbated when you work with an application that creates very large temporary files, such as an image-editing program. And remember that tossing files into the Recycle Bin (which is what happens when you manually delete a file) doesn't actually remove them from the disk. The Recycle Bin will fill up to the maximum size that you've configured it to support. (It has a default of 10 percent of the available disk space.) So using the Disk Cleanup tool on a regular basis is a worthwhile activity for most users. I've recovered as much as 10GB of storage by taking the tool's suggestions for what files should be removed. A quick look at the computers in my office found an average of about 2GB of space being occupied by stuff that I had moved to the Recycle Bin. So good garbage collection is an important part of the storage-space recovery process.

The last culprit is the least known: system restore points. By default, Windows XP creates a system restore point at least every 24 hours; possibly as often as every 10 hours when the system is running. Depending on the system configuration, the size of these restore points can easily be 2MB to 5MB or even larger. This might sound like much, but consider how much space restore points take up on a computer that is left running all the time. Although, it might not seem like much space on a desktop system with a 250GB hard disk, the restore points will take up a much greater percentage on a notebook with only 30GB of disk space, though the default maximum is 12 percent of the disk size.

Other factors can affect available hard disk space on Windows computers--for example, a system that experiences a large number of blue screen lockups and is configured to do a kernel dump to a file after each crash or a computer that you've configured with very large page files, which can create storage problems--but they are much less common than the three culprits outlined here. Good storage housekeeping, along with understanding the actual available storage space your drive started with, will clear up most concerns about "missing" storage space.