Download/PurityScan is classified as a Downloader.
Downloaders are applications that connect to a remote host to stealthily retrieve files.
The Fortinet Anti-Spyware Research Team is currently in the process of creating a detailed description for this Spyware.
For additional information about Downloaders, and how Fortinet classifies Spyware threats please see Fortinet's paper in PDF format:
Spyware Classification and Terms.