Windows will always be installed on a partition, to install an OS you always have to partition the hard disk, even if it is only a single partition; you would like to know the advantage of installing windows on a seperate partition and storing your data on another partition.
There are many advantages. In case of a system crash, you can format the Windows partition without having to worry about your important data (which you should save on the other partition).
It makes organizing your files easier.
You could save your torrents and partially downloaded torrents (or files you download with other P2P programs) on the second partition, and continue downloading after you re-installed your P2P client on your freshly installed operating system.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....