Just reinstalling the OS from scratch, from a windows CD, will free up all the stuff. Unless you actually want to zeroise it for security reasons.
Most PCs contain a Recovery Partition that allows you to get it back to factory condition.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC