@greedy kite & ixion
Pressing
Alt &
F4 together should stop any program running on Windows, such a browser which has run across something nasty. The command simply kills the program & does no harm elsewhere.
An anti-virus program such as AVG incorporates with Firefox to block attacks. Usually does the job very well, but the "nasty boys" out there are ingenious. If in doubt, kill it immediately.
Never, ever click on anything that wants to scan your computer, improve it's performance, etc.
There are plenty of legitimate programs available that will do various house-keeping functions. Go to their actual sites or try something like Cnet (Google it) to preview them.
Re Java, see my post @5
http://www.crosswordsolver.org/forum/457537/offset/4