Try reinstalling Java
http://java.sun.com/j2se/1.5.0/download.jsp
If the latest version doesn't help, go back to an earlier version or try Microsoft Java.
runescape.com has a Customer Support section with a 'Bug Reporting System'.
You need to have an account to enter this section, so I couldn't check to see if it's a known problem.
Any luck with the antivirus/malware?