See the other FAQs in "Desktop Software > PC Operating Systems > Linux (client/desktop)", especially the one entitled "An easy way to dual boot Windows and Linux (on a empty HDD)" FAQ619-3492. This is an excellent starting point!
If your motherboard has onboard features such as LAN/Ethernet port, sound card, etc., and they are to be used and not replaced by PCI-cards, and you are new to Linux and the Debian distribution of Linux, forget doing the orthodox install via the CD-ROMs, and just get Morphix or similar Knoppix-type pre-configured Debian CD-ROM and use that. The regular install of Debian "Woody" kernels 2.2/2.4 doesn't seem to readily find onboard components.
If you are not going to completely reformat/partition the hard drive, and seek to keep the present OS as a boot-up option, you'll need to use a product such as Partition Magic or similar partitioning software that is capable of reducing the size of a partition while preserving its data.
If going with FAQ619-3492, and installing Debian, note that Debian will not ask for the partitions in order, so be aware that "/swap" will be hda6, "/boot" will be hda5, and "/" will be hda7.