Do you know that you can define up to four partitions in your hard
drive? If you have only one partition at this time, I'd recommend
making at least another partition where you can install Zipslack.
Now before you get deeper into partitioning, make a backup (maybe
a full backup) so you can always recover your old data in Windows.
You do not need to have an extended partition to install/run linux.
It can be done, but in your case, you are better off using another
primary partition since you seem to have the space available, right?
Hope this helped