I've seen many questions, and even more incorrect or incomplete replies about missing disk space in Vista. You got a 160GB hard drive and you've only installed a few programs, but you're missing a LOT of disk space! Where did it go? There are numerous possible reasons why your disk is getting used up -- sometimes it's being used by useful services, and other times it's just being wasted.
So where does the space go?
Shadow Copy (also called Volume Snapshot Service or VSS) is a feature in recent versions of Microsoft Windows that allows taking manual or automatic backup copies or snapshots of a file or folder on a specific volume at a specific point in time. It is used by NTBackup and the Volume Shadow Copy service to backup files. In Windows Vista, it is used by Windows Vista's backup utility, System Restore and the Previous Versions feature.
This service can be very useful, and is a good idea to keep it enabled. However, it does eat disk space. A LOT of disk space. By default it allocates %15 of the disk to store it's data. On a 160GB disk, that's 24GB! A lot of people have been noticing that after a few weeks of using their new system, free space seems to shrink daily. This is because the space is not allocated right away, only when it's needed. It will stop when it reaches %15. At that point, it will delete older versions to make room for newer ones. For all the details about Shadow Copy, read this article at ZDNet for a really good explanation.
However, computers are binary systems, and measuring in multiples of 1000 isn't the way they do things. The closest thing we have in binary is 1024. So, for a computer, 1K = 1,024, 1M = 1,048,576 (1024 * 1024), and 1G = 1,073,741,824 (1024 * 1024 * 1024). As a result of this, a computer thinks that 1GB is bigger than what a person typically refers to as 1GB (a difference of 73,741,824 bytes). If we take our example of a disk that's advertised as 160GB, and divide by what a computer thinks is 1GB, we wind up with: 160,000,000,000 / 1,073,741,824 = 149.012, which is what Windows says is the drive capacity.
This measurement makes it seem like the drive is smaller, which is the reason I call this "marketing". Everyone wants to make their drives seem bigger, so they use the larger number, even if it's not exactly accurate.
Because of this confusion, new standards of measurement have been devised to help clear this up. Officially, the term "megabyte" refers to 1,000,000 bytes (1000 * 1000), and the term "mebibyte" refers to 1,048,576 bytes (1024 * 1024). The abbreviation for "megabyte" is "MB", like you're used to, and for a "mebibyte" it's "MiB". Notice the "i" in there. It's subtle, but important to make the distinction. You probably won't see these units in use by large companies for a while, but it's something you should be aware of anyway. See mebibyte for more information.
Here's a table comparing the "marketing" size vs. the computer size for some typical drive sizes:
|80 GB||74.51 GiB|
|100 GB||93.13 GiB|
|120 GB||111.76 GiB|
|140 GB||130.39 GiB|
|160 GB||149.01 GiB|
|200 GB||186.26 GiB|
|250 GB||232.83 GiB|
|300 GB||279.39 GiB|
|320 GB||298.02 GiB|
|350 GB||325.96 GiB|
|400 GB||372.53 GiB|
The partition can be anywhere from about 5-10GiB, but it can vary. There are ways to get rid of it and reclaim that space, but never do so unless you have burned the "recovery discs" onto some DVDs first. The process to make recovery discs should be detailed in the documentation of your system. Once that is done, you can delete the partition, then expand your C: partition to use that space.
Another program that does this is Microsoft Office. When you install Office, it copies the entire contents of the disc onto your hard drive. Unfortunately you cannot remove it, as these files are needed when you do an Office update.
One way you can track down what's taking a lot of space is by using an open source tool like windirstat. Don't go deleting stuff you don't know what it is! Many are the stories of people who reorganized "this c:\windows thing" and for some reason their systems didn't boot anymore!
One principle to keep in mind is that free space can be considered wasted space. If you don't need that space now, let Windows use it for temp files, indexing databases, volume shadow copy, etc... so you can get the benefit of those services. Only when you need that space should you worry about deleting those things.