what partition should I use for software (e.g., apache, perl, c++ compiler)?
 

Hi. I'm about to partition my hard drive to use Slackware & Windows. Before doing so, should I use a separate partition for software like Apache? What about any other software I decide to install in the future?

There is really no need to, the standard is three partitions one for the root filesystem, one for /home, and one for swap. Even then, there is really no rule that says /home should be separate, it just helps with upgrades.