2 Primary Partitions

I'm new here! Just built my first PC over the weekend. I have a question about partitioning. When I installed my OS, I set up my first primary partition to be 100GB and left the remaining 400GB to be partitioned later. After I loaded and patched everything, I went into Disk Management and formatted the remaining unallocated space as a Primary partition. It wasn't until after, that I realized I should have selected Extended. The reason I say that is because I'm only using this partition for data storage. From what I've read (and think I understand) is that people generally setup a second primary partition if they want to load another OS. Is that right? Since I have not put anything on this second partition, can I delete it and then reformat again (this time selecting "Extended" ) or is it okay to leave it as is? I wanted to make sure that if I deleted the second partition, that it would return to "unallocated space" and not affect what i've already installed on drive C:. By the way, I have Windows XP pro as my OS.

Thanks!
Liz