Your .htaccess should look like this:
AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wordpress/index.php [L]
</IfModule>
# END WordPress
If the php5 handle is not working for you then change it back to php4
Making it read thus:
AddType x-mapp-php4 .php
AddHandler x-mapp-php4 .php
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wordpress/index.php [L]
</IfModule>
# END WordPress
At this pont your blog will only work if the current php version is higher than 4.1.3 - if not ask your host to upgrade you to the latest php4 version.