I don't think that putting antivirus software on your computer will have affected its ability to run java applets (a jigsaw in this case). Java VM (virtual machine) is being phased out by Microsoft but it is still supported by them until the end of 2007 for those who already have it installed. It should be installed on your computer because you are on W.98. Try this:
* Open IE
* Go into: Tools > Internet Options.
* Click the 'Security' tab.
* Click on the 'Internet' icon.
* Click on the 'Custom Level' button.
* Scroll down the list to the 'Scripting of Java applets' heading.
* Make sure that 'enable' is selected.
* Click OK and OK again to close the box.
* See if the jigsaw site works.
Did you install Java for Firefox? If so, you could also try using that:
* Go to the Control Panel.
* Find the Java Plug-in icon and double-click it.
* Select the 'Browser' tab.
* Check the box that says 'Microsoft Internet Explorer.
* Click the 'Apply' button.
* Close the box.
* See if the jigsaw site works.
Information about Microsoft VM