Viruses and spyware can be slithery little pests. Not all typical anti-virus and anti-spyware software may remove them. Nowadays, there are trojans, bug, rootkits, and other infections that may be considered both spyware and viruses. So, with different variants of infections running into the gray area of what it actually is, the security software to remove them has been trickier.
We’ve seen as of late a new trojan that infects the WindowsSystem32 folder. This trojan 
will cripple the Windows Installer so you cannot run some programs, such as itunes, as well as not being be able to run Windows updates. Many programs will not remove this trojan, including: Norton Anti-virus, Malware bytes, Spybot Search and Destroy, Hijack This, and AVG Anti-virus.
Combofix has been able to remove this tough-to-kill trojan. Combofix is a DOS program that runs within Windows and removes infections that may be found in the WindowsSystem32 folder. Combofix is effective since it will actually delete these infections that cannot be manually removed.
New trojans cannot be removed manually as they are write-protected and are just too 
smart to be simply deleted. So, Combofix has come as a savior for removing trojans. You’ll need to make sure to disable all current anti-virus, anti-spyware, and firewalls while you’re running it to prevent startup problems afterwards.