Hi nerdgirly -
I'm going to proceed on the assumption you want to know what you asked rather than advice on how and why to do something different...
That said,
1. open a dos window (run CMD or search and double click CMD) and type format <D>: /FS:FAT (replace <D> with the drive letter of the SD and no < >'s) - this should work for any version of windows released in the last decade.
or 2. Run DISKMGMT.MSC in Windows7, Vista, or XP, scroll down until you see the SD drive (probably the last one in the list), right click on the drive letter ( or anywhere on or under its blue bar), right or left click format from the list and select FAT for file system and click OK