So it's time to bite the bullet. Instead of just experimenting with a live CD, you've decided you want to go with a full, permanent installation. But that doesn't mean you have to immediately stop using your current OS, if you don't want to.
What you can do, is split your hard drive into partitions, keeping your current OS on one partition and installing a Linux distribution on another. That way, you can keep both systems on your machine, and then just choose when you power up which OS you want to boot.
This sounds simple, but is slightly complicated by the different formatting systems which the systems like to use for their hard drives. We've already come across this when using Knoppix to access Windows' NTFS file systems - it can read the drives OK but the writing support isn't quite reliable, because the nitty-gritty details are kept secret. And when you're talking about writing to hard drives, reliable is exactly what it has to be, every time. So in practice you're likely to want to keep your NTFS partitions for use by Windows, but make the Linux partition in an exclusively Linux format like ext3.
That means that the Linux will be able to read from the Windows drive, and will of course be able to read and write to its own drive. But Windows of course doesn't understand the Linux format drive, (and makes no effort to!), so it makes sense to have a third partition which is supported by both. This would be formatted as VFAT, which both Windows and Linux can write to and read from, and would allow a common area for swapping files to and from systems.
Windows' support for managing the partitions is surprisingly poor, Unless you buy additional tools, you can't resize your partitions to make room for the new system. (It's also unable to install itself alongside another OS, so for a dual-boot system, the Windows system has to go in first, and the more capable system installed alongside afterwards).
Apparently the distribution install will be able to handle the resizing, but I wanted to create the VFAT partition from within Windows so that the chances of it recognising it were greater (I was already confident that Linux would recognise it!). So my strategy was to resize the partitions using QTParted within Knoppix, create the VFAT partition from within Windows, and then install Linux on the unformatted spare space.
But of course these are pretty fundamental changes to the hard drive, including reformatting, so the first thing to do before anything else is to back all the personal files onto CD in case they go astray. And the second thing to do is delete as much unwanted stuff as possible from the drive to be resized, and do a defrag to try to persuade the system to reorganise the files to the beginning of the partition. Of course it may or may not take these hints!
The next thing to do is try to decide sizes - this depends on how big your hard drive is, how much is already taken up with Windows, and how much you plan to use both systems side-by-side. I have a 60GB drive, so as a first go I decided to reduce the Windows partition (which was taking up the whole drive) down to 20GB, make a 5GB VFAT partition, and have the rest for Linux.
Reducing the size of the Windows partition was easy, as it was a relatively large and very new hard drive, with little fragmentation. Hence everything on the drive was already inside the first 20GB and there were no problems. I immediately rebooted into Windows to make sure it was still ok, It did complain that the drive was "dirty", but after this minor grumble it was happy and showed me my now shrunken 20GB C: drive.
Next step was to create the VFAT partition, which I did using the Disk Management tool (Control Panel - Administrative Tools - Computer Management - Disk Management), specifying 5GB and FAT32. At the first go I specified a Primary Partition, which was wrong because there's only a limited number of Primary partitions and the Linux distribution couldn't then create its own Primary partition. Instead the FAT32 partition should be specified as an Extended Partition.
With that step successfully completed, Windows now sees a main C: drive and a new E: drive, so we're ready to go with the installation of the Linux distribution. I chose Mandrake 10.1, so went ahead with the installation.