Each partition used by Windows gets assigned a drive letter, usually C:, D:, E: etc. You can put other operating systems, such an Ubuntu Linux, in other partitions.

Back in the days of DOS, allowing 4 partitions per disk seemed generous. Oddly that limit is still with us in the form of a maximum of 4 primary partitions per disk. If you need more, which you nearly always do, you designate one or more of your four primary partitions as an extended partition, then split it up into many subpartitions sometimes called secondary partitions or by old timers as logical volumes.

You must mark one of your four primary partitions as active. This means it’s operating system is the one that gets booted on power up. Partitions that that contain a potentially bootable OS are marked bootable. Sometimes the boot invokes a tiny program called a boot manager that lets you dynamically choose which OS you want to boot. In theory you can’t boot a secondary partition, but in practice you can, by using a boot manager roosting in a primary partition that redirects to the secondary one.

You can reassign drive letters and examine your partition structure in Control Panel ⇒ Administrative Tools ⇒ Computer Management ⇒ Disk Management. Windows comes with extremely primitive command line tools FDISK and DISKPART to manage partitions. Pretty well you have to decide when you first set up the machine what your partitions will be then leave the intact ever after. About the only thing you can do is convert a FAT32 partition to NTFS.

However with utilities like Acronis Disk Director or PartitionMagic you can create, delete, grow, shrink, shuffle, copy, split and merge partitions.

Since you can’t easily change a Windows partition while Windows is actively using it, you might find Linux partitioning utilities easier and safer to modify Windows partitions and Windows partitioning utilities easier and safer to modify Linux partitions.

Microsoft invented a proprietary partitioning scheme called dynamic disk that the partitioning utilities will not touch. Happily the home versions of XP/Vista don’t support it, so you don’t have to worry about it if you are using a home edition.

On my Win2K machine I have four partitions: