There is still a Flash vulnerability out in the wild that can cause you to inadvertently click and install software that could damage your computer or at the very least serve up advertisements that you don't need and cause your computer's performance to degrade.
Flash is a piece of software that runs on most computers these days to play movies or videos from places like youtube.com or cnn.com.
The attack works though the advertisements on major websites by pasting a url into the clipboard of a target computer. If the link is clicked on it takes you to a website and tries to force the target to install software that is suppose to fix you computer. What the software really tries to do is capture personal information and install advertising software for spamming purposes.
Please take a look at the screen shots and become familiar with the what this attack looks like so you can avoid it on the internet.
http://malware-test-lab.blogspot.com/2008/08/analysis-of-mystery-eb-atta...
If you spot this attack while surfing the net it is recommended that you stop what you are doing, save your work, restart the computer immediately and run a virus scan. You might not be able to close your browser as the vulnerability tries to prevent you from doing that. Restarting the computer will ask you to force close or end tasks what ever is running.
Adobe, the new developers of the Flash product, are currently working on the problem. Updates will follow in the next few weeks. Until then you might want to either uninstall Adobe Flash, or configure your browser options to disable remote clipboard operations.