These don't do too much damage yet, mostly deleting characters from documents and printing strange words in documents. The potential for worse is there. There is now more concentration on system vulnerabilities and less virus writing. In 1989 the DOE issued 8 advisories for vulnerabilities and 5 for viruses compared to 89 vulnerabilities but only 3 viruses and worms for 1998. This is a malicious java applet that enters your machine free of charge, courtesy of your browser.
Damage can range from file destruction to information gathering to control of PC functions.
A nice introductory page on the Internet. It includes virus and credit card information on the net. Use passwords that contain lower and upper case letters, and numbers. Change passwords frequently. Encrypt sensitive or critical data.