If you want to get rid of Windows completely and go for LInux:
in the installer it will ask you whether you would like to use whole hard drive or create a partition.
If you want to size down Windows and put Linux on the free space:
If you have a pre-Windows NT (95/98/ME) machine then RH9's installer will size it down (like Panos said).
If you have a Windows 2000/XP machine and you want to size down the Windows space, you may need to choose Mandrake Linux, because that has NTFS resizing (NTFS is Windows 2k/XP's file system).
A general note: If you have never seen Linux before, I suggest you dual boot for awhile, some things are different in Linux, and its nice to be able to go back to what you know once in awhile. I'm not saying Linux is hard, because these days its not: but there are differences.
And in case it ever crossed your mind, dont worry about not being able to get into Windows anymore after you install Linux, all Linux vendors use a boot loader, either LILO (the Linux Loader) or GRUB. Both LILO and GRUB allow you to boot into other operating systems and just about every vendor today autodetects Windows partitions and puts them on. You may be surprised however, because it is usually detected as 'DOS'. Don't worry, it's still Windows
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