RewriteEngine On
RewriteBase /product/blog/

RewriteRule ^c-(.*)$ catpost.php?id=$1 [L]
RewriteRule ^a-(.*)-(.*)$ archives.php?month=$1&year=$2 [L]

RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteCond %{REQUEST_FILENAME} !-f [NC]
RewriteRule ^(.*)$ viewpost.php?id=$1 [QSA,L]