CaSIs is short for Common and Stubborn Infectors. These are malicious programs (Viruses, Worms, Trojans, etc.) that are notoriously difficult to detect and to remove by regular anti-virus programs. These malicious programs often have the capability to disable your computer or your anti-virus programs.
Good examples of these infectors are:
Trojan.Win32.Small.wv (Medichi & Medichi2)
Worm.Win32.AutoRun.dkk (Ahsan virus)
If one of the above nasty infectors infected your computer, you will not be able to install any of the well-known Antivirus software like Kaspersky, Mcafee, Norton, AVG, Panda... (and about 135 more different AVs!) and please, don't try to use Safe Mode to remove them manually because those infectors will disable "Safe Mode"!