PUP or PUPs is a term used to describe unwanted programs such as Trojans, spyware and adware, along with other malware which may compromise your privacy. Some antivirus and PC security software packages, like McAfee VirusScan, will scan for and protect your system against PUPs. The term PUP was first used by persons at McAfee's Avert research lab to avoid any legal issues that may arise from calling these types of applications "spyware".

McAfee VirusScan
McAfee VirusScan products offer protection against PUPs.

Webopedia's "Did You Know" Section
The Difference Between Adware & Spyware.