The UBB does not support subcategories, you can however use a category of "Operating Systems" and have a forum of "Windows", another of "Linux", etc.
If you want it to show up like at
UGN Security you will have to turn category viewing on (CP -> General -> Primary Tab -> Forum Summary Options dropdown, then tick the "Enable Categories" tick and submit).
Additionally, i don't believe there is a mod over at
UBBDev for subcategories however it is usually the best place to start for UBB Modifications.