Install ISAPI Rewrite from http://www.isapirewrite.com there is a 30 day trial or a free lite version.
once installed edit the httpd.ini file and add the line:
RewriteRule /forum/archive/([^/.?]+\.html) /forum/archive/index.php\?$1 [I,L]
Now edit the index.php and global.php in the ARCHIVE FOLDER.
eg: /forum/archive/
Find and replace 'index.php?' with ''
Save both files.
Thats it - browse to your archive pages:
http://www.mydomain.com/forum/archive/
And you have very friendly google links!
NOTE: Requires Windows Server and IIS (Internet Information Services) this will not work on Apache (although there is a very good mod_mime hack to do the same thing on Apache)