Note that the point of DBAN is to overwrite every possible spot on a disk, so that the data that was previously stored there is no longer available. On your hard drive, there is a "table of contents" so to speak, which tells where on the disk each file is found. For example, this table might say that file "mystuff.txt" is stored in locations #7845-#7849. When you delete mystuff.txt in Windows, the table of contents is updated to show that locations #7845-#7849 do not contain data (new data can be written there). Even though those locations are marked as available for writing, the actual locations still contain whatever data they had before. Even though Windows thinks the file is gone, the 1's and 0's that made up the data are still stored on the disk.
Along the same lines, FDISK will delete the "table of contents" for the whole disk structure. All your partitions will be gone, and you will be starting fresh from a "clean" disk. But again, the data is still there, the space is just marked as being available to be written.
DBAN overwrites all this "available" space (actually ALL disk space) with garbage. None of the remnants of your data will remain on the disk afterwards. It is used when disposing of a drive, so that none of the old data can be recovered by someone else.
If you plan to still use the computer, you might as well just format or fdisk the drive. This will give you a "clean" drive to start your install from. The remnants of your old data will still be on the drive, but they'll be overwritten by the new Windows installation and anything else that gets written to the disk. Reusing the disk does the same thing as DBAN - it overwrites the data remnants with other data. The only difference is that your new data may be stuff you don't want left on your disk (so when you really want to get rid of the drive, you'll need to use DBAN), while DBAN fills it with meaningless garbage.
Reinstalling Windows after DBAN (or a format/fdisk) is exactly the same as installing it on a brand new hard drive. You'll have to set up the file system (NT-based Windows versions let you do that as part of setup after booting off the CD), install Windows and any needed drivers (like ueivar said), and your programs and any data you backed up. DBAN (and format/fdisk) doesn't have anything to do with your BIOS, but you may want to change the settings in your BIOS to boot off the Windows CD before booting off the hard drive, just to make the install easier.
To sum all that up, DBAN doesn't affect anything about the way your computer or disk work. DBAN simply overwrites possible data remnants with garbage so that your old data can't be recovered. Note that overwriting all the programs and data on your disk with garbage (using DBAN or otherwise) will
affect your ability to use your computer though. =)