Why does Netscape 4.7 require 100% of CPU usage and lock up my
computer for a short period of time when searching many sites,
while Internet Explorer uses about 45% searching the same sites and
doesn't cause a problem?

I used to have this problem a while ago. It turned out to be one of
two things. Either, DNS lookups were going wrong, or the page being
loaded involved a lot of tables. Why not have a look at the latest
version of Phoenix, a web browser based on Mozilla, but which is
rather more lightweight!

http://www.mozilla.org/projects/phoenix/phoenix-release-notes.html