The Computer Trainers

Question: Why are there viruses?

Short Answer: Money

Long Answer:  Most virus, malware and spy-ware infections around today want your money - underlying that are a few broad categories that I see regularly.

1. Getting your money: pop ups, pop under, or annoying balloon tips that keep reappearing in the system tray usually offering a security package to fix a perceived problem.  They can hijack your desktop and your web browser. 
2. Sending out spam: Hijack of your computer solely to send out spam. You won’t normally see this one.  You could incur excess data charges from your ISP or you find your Internet access slowed to a crawl.  Spammers get money from the ‘advertisers’.
3. Keystroke Logging: Sit quietly in the background monitoring keystrokes More often nowdays phishing emails are used to gather credentials. 
4. Ransomware: Encrypt your files then hold you to ransom in order to recover your data - usually around $400-500

List of known Rogue security software.
Advanced Cleaner AKM Antivirus 2010 Pro AlfaCleaner Alpha AntiVirus ANG Antivirus Anti Virus 2010 AntiMalware Antimalware Doctor AntiSpyCheck 2.1 AntiSpyStorm AntiSpyware AntiSpyware 2009 Antispyware 2010 AntiSpyware Shield AntiSpyware Soft AntiSpywareExpert AntiSpywareMaster AntiSpywareSuite Antivermins Antivir Solution Pro Antivirus 2008 Antivirus 2009 Antivirus 2010 Antivirus 360 Antivirus 5 Antivirus 7 Antivirus 8 Antivirus 8 Antivirus Action AntiVirus Gold Antivirus IS Antivirus Live Antivirus Master Antivirus Pro 2009 Antivirus Pro 2010 Antivirus Scan Antivirus Soft Antivirus Studio 2010 Antivirus Suite Antivirus System PRO Antivirus XP 2008 Antivirus XP 2010 AntivirusGT AV Antivirus Suite AV Security Suite Avatod Antispyware 8.0 Awola BestsellerAntivirus Browser Defender ByteDefender ByteDefender Security 2010 Cleanator ContraVirus Control Center Core Security Cyber Security Defense Center Desktop Security 2010 Digital Security Disk Defrag	Disk Doctor Doctor Antivirus Doctor Antivirus 2008 Dr Guard DriveCleaner EasyFix Tools EasySpywareCleaner eco AntiVirus Error Expert ErrorFix Errorsafe ErrorSmart Flu Shot 4 Green Antivirus 2009 Hard Drive Diagnostic HDD Plus HDD Rescue HDD Rescue HDD Scan IE Antivirus IEDefender InfeStop Internet Antivirus Internet Antivirus 2011 Internet Security 2010 KVMSecure Live PC Care Live Security Suite MacSweeper Malware Bell Malware Defender Malware Defense MalwareAlarm MalwareCore MalwareCrush MaxAntiSpy Microsoft Anti Malware MS AntiSpyware 2009 MS Antivirus MxOne Antivirus My Security Engine My Security Shield My Security Wall Navashield Netcom3 Cleaner PAL Spyware Remover Paladin Antivirus PC Antispy PC Antispyware PC AntiSpyWare 2010 PC Clean Pro PC Defender Antivirus PC Privacy Cleaner PC Scan and Sweep PC-Doctor PCPrivacy Tools PCSecureSystem Perfect Defender 2009 Perfect Optimizer PerfectCleaner	Personal Antivirus Personal Security PersonalAntiSpy Free Privacy Center Protection Center PSGuard Quick Defragmenter Rapid AntiVirus Real AntiVirus Reg tool RegAlilve Reggenie Antivirus Registry Defender Registry Great Registry Patrol Registry Winner Remove Disk Doctor Remove Hard Drive Diagnostic Remove HDD Fix Remove HDD Plus Remove Memory Fixer Remove Palladium Pro Remove Personal Internet Security 2011 Remove System Tool 2011 Remove Win Defrag Remove Win HDD Safety Alerter 2006 Safety Center o SafetyKeeper SaliarAR SecureFighter SecurePCCleaner SecureVeteran Security Center Security Essentials 2010 Security Master AV Security Scan 2009 Security Suite Platinum Security Tool Security Toolbar 7.1 SiteAdware Smart Antivirus 2009 Smart Engine Smart HDD Soft Soldier Spy Away Spy Tool SpyAxe SpyCrush Spydawn SpyEraser SpyGuarder SpyHeal Spylocked SpyMarshal SpyRid SpySheriff SpySpotter Spyware Blaster Spyware Cleaner	Spyware Doctor Spyware Protect 2009 Spyware Protect 2009 Spyware Stormer Spyware Striker Pro Spyware X-terminator SpywareBot . SpywareGuard 2008 SpywareQuake SpywareSheriff SpywareStrike SpyWiper SysGuard Sysinternals Antivirus System Antivirus 2008 System Defragmenter System Live Protect System Security System Tool 2011 SystemArmor SystemDoctor TheSpyBot ThinkPoint Total Secure 2009 Total Win 7 Security Total Win Vista Security Total Win XP Security TrustedAntivirus UltimateCleaner Ultra Defragger Virus Locker Virus Response Lab 2009 VirusHeat VirusIsolator VirusMelt VirusProtectPro VirusRanger VirusRemover2008 VirusRemover2009 VirusTrigger Vista Antimalware 2011 Vista Antispyware 2010 Vista Antivirus 2008 Vista Internet Security 2010 Vista Smart Security 2010 Volcano Security Suite Win 7 Antivirus 2010 Win Antispyware Center Win HDD Win7 Antispyware 2011 WinAntiVirus Pro 2006 WinDefender Windows Police Pro Windows Protection Suite WinFixer WinHound Winpc Antivirus Winpc Defender WinSpywareProtect WinWeb Security 2008	Wireshark Antivirus WorldAntiSpy XJR Antivirus XP AntiMalware XP AntiMalware 2010 XP AntiSpyware 2009 XP AntiSpyware 2010 XP Antivirus 2010 XP Antivirus Pro 2010 XP Defender Pro XP Guardian XP Internet Security 2010 XP Security Tool XP-Shield Your Protection Zinaps AntiSpyware 2008
Source: http://en.wikipedia.org/wiki/List_of_rogue_security_software