Wikipedia
Trojan:Win32/FakeSysdef, originally dispersed as an application called "HDD Defragmenter" hence the name "FakeSysdef" or "Fake System Defragmenter", is a Computer Trojan type of computer virus targeting the Microsoft Windows operating system that was first documented in late 2010.
Win32/FakeSysdef manifests as one or more of an array of programs that purport to scan one's computer for hardware failures related to system memory, hard drives and system functionality as a whole. They scan the computer, show false hardware issues, and present a remedy to defrag the hard drives and fine-tune the system performance. They then request, from the user, a payment in order to download the repair update and to activate the program in order to repair these contrived hardware issues.
The fictitious scanning program detects one or more of the most widespread varieties of risks prevalent on the internet today. Everyday numerous fake antivirus and security applications are published and released to unsuspecting end-users via a large assortment of distribution channels. Many times such software turn out to be clones of each other – developed from the same code base but packaged with a unique title and design through the use of a "skin".
The branding strategy may look legitimate to computer users as the names are usually a combination of technical words such as "HDD", "Disk", "Memory" and action words such as "Scanner", "Defragmenter", "Diagnostics", "Repair", and "Fix".