Last summer I had a virus that took over my laptop and wouldn't let me do anything. It kept showing porn

and then tell me that I had to buy a particular security software. If I managed to turn off the computer and turn it back on it would recommend a different software package. I had to take it to a local computer repair shop and have the entire disk backed up and cleaned up. It cost me $150! Now I am very cautious and update the security software immediately if I haven't been on the laptop for a couple days. So far there has been no further problems.
ETA: The virus I had was ANTIVIRUS 2008. Following are symptoms from the document that the repair shop gave me:
SIGNS OF AND ANTIVIRUS 2008 INFECTION:
The following symptoms can be seen after the Antifirus 208 virus has successfully infected your computer:
1. "VIRUS ALERT" appears inthe system tray (lower right hand corner of your desktop)
2. The computer's desktop background changes to a red hazard alert or a blue background with a yellow alert in the center
3. The computer's screen saver changes to either the "Blue Screen of Death" with a random error code that changes or animated bugs crawl around the screen
4. The Start Menu does not list "Programs", "My Documents", "My Computer", "My Recent Documents", "Search", "Help", "Control Panel" or "Run".
5. Pop ups of random rogue anti-spyware programs appear. The pop-ups displayed are for rogue anti-spyware programs like Antivirus 2008, Antivirus 2009, Antivirus XP 2009, XP Antivirus Protection, Spy-Shredder, WinSpywareProtect, and others.
6. Windows Explorer closes by itself or displays an "Illegal Operation of System" page
7. Computer performs sluggish or it ultimately crashes. Afterward it only "loop boots" over and over
8. The Antivirus 2008 program will hide inthe _"Program Files" folder located in C:\druve directory ina sub-folder named something like "XP Antivirus 2008" or "Antivirus XP"
If you see any signs or activity as we have described above, it is very likely you computer is infected with the Antivirus 2008 virus. Shut down your computer immedieately.
DO NOT SUBMIT YOUR CREDIT CARD INFORMATION.
Contact your computer professional to have Antivirus 2008 removed from your computer. Once a computer has become infected with this program, specialty software will have to be used before your computer becomes completely disabled.