As far as your pc, I can't really say, you might want to call microsoft
support for your windows questions.

As far as your linux box (I gather you meant ubuntu 8.10, as there was
no January 2008 release) If you are using a firefox package from the
ubuntu repository, it is installed system wide, and individual users can
not modify it. To upgrade it, you need to upgrade it through the
repository. See this web page for details.

webupd8.blogspot.com/.../...tu-repository-deb.html

Then firefox will be updated quite often - several times a week.

If OTOH you want to discard the ubuntu firefox package and do it all
yourself, then just go into the synaptic package manager
(system->administration->synaptic) and remove firefox.

After removing the firefox package, you can download a firefox tarball
from firefox.com, and do things the old school way. Be sure and
recursively change the ownership of the installed firefox directory to
your user id, which will allow you to do the updates from firefox.com as
a normal user. Do remember to manually create and update all the
symlinks etc for your plugins. Other users on the system may or may not
be able to use firefox depending on how you set it up.