(And thank you, rainbow-ga, for allowing me to post this as the answer.)
First of all, go to this website:
Spambayes on Sourceforge
and download the Spambayes client from here:
Once downloaded, close Outlook and then install Spambayes. Once
Spambayes is installed open Outlook (view the read me files if you
wish) and you will be presented with a screen. Choose "I haven't
prepared for Spambayes at all" then choose "I will pre-sort good and
spam messages, and configure Spambayes later"; then choose finish.
Now you have access to your whole mailbox again. I would advise you to
move all mail into a folder called "unsorted" (the reason why will
become apparent soon). Once done, move at least 100 junk mails (or as
many as you can be bothered to move) into the JUNK EMAIL folder and
100 NON SPAM (or as many as you have) into the INBOX. Once done click
on Spambayes within Outlook, then click Spambayes manager. You should
now have a new window pop up. Click the Training tab and click BROWSE
next to "folders with known good messages" and choose inbox as the
good messages folder. In "folders with spam or other junk messages"
click BROWSE and choose JUNK E-MAIL or your spam folder. You should
now have certain spam in the spam folder and not spam in the inbox and
those messages you could not be bothered to filter in the unsorted box
(doing this spambayes will not get confused and incorrectly mark junk
as good and vice versa).
Once this has completed click on the button marked "start training".
Your machine will now spend a while catagorising the mails you have
decided to learn on. Go make a drink and come back in 10 minutes.
Once this is done click the GENERAL tab and then put a tick in "enable
spambayes" box. This will enable Spambayes for future mails.
Now comes the time of testing.
Click close and click Spambayes within Outlook and then filter
messages. In the "filter the following folders" choose your "unsorted"
folder and ensure that "perform all filter actions" is highlighted and
then click "START FILTERING".
Your unsorted folder should be filtered for spam and good emails. Now
is the time to certainly go make dinner for yourself as this is going
to take a long time, especially with 4GB of emails :)
If you need any additional help or clarification of my answer before
rating, let me know and I'll be glad to assist you.