If you want to zap the existing partitions before starting your new install...
Boot from a live CD such as Ubuntu or Knoppix
Open a terminal window and do the following
sudo fdisk /dev/sda
The "1 <enter>" step may not be necessary - fdisk may not ask for which partition to delete if only one is left - have not done this recently and am writing from memory.
These steps will...
delete partition 2 (sda2)
delete partition 1 (sda1)
write the changes to the disk
THIS WILL WIPE OUT ALL DATA ON THE DISK!!!!!
As Agrouf said - the install process for most distros will include steps to partition the drive. Chose "manual" and then delete the existing partitions and create new ones.
If you for some reason want to sanitize the disk before installation - do a google for DBAN. Download and burn to a CD, boot the computer from the CD, follow the on-screen prompts and wait a few hours. The drive will be cleaner than when it left the factory