Unfortunately, if you only have one local disk, the ESX installation will reformat and repartition that drive. There is some good news though!
Instead, you can download ESXi, which is a free hypervisor and is considered "installable". So, you can put the ESXi software onto something like an external HD or a USB stick and you will be able to boot your ESXi server using whichever external drive you select. This will also allow you to keep the partitions on your local disk in place while you use that disk's remaining space for your VM datastores.
Here is a great link from VMware that shows you exactly how to do it, enjoy!