If you have a pre-2001 model, you may need to install a suspension update service kit.
There were a lot of front suspension issues with pre-2001 S40 & V40 models. Volvo made some changes to the front suspension in 2001 and offered a service kit for pre-2001 models that updated the strut & spring mounts to the 2001 style.
Cars that have already had this update can simply use the 2001-2004 style strut and spring mounts.
The most obvious way to tell if the update has been done is that updated cars have an accordian style strut bellows