A decade ago, nearly all software introduced to a machine was introduced from a floppy. It was a simple matter to determine the immediate source of that software, and to scan it with an anti-virus scanner. Today, nearly all software is introduced to a machine via the Internet. The transfer process might reveal the overall setup package, but not its components. Even the size of the basic component cannot normally be determined with precision.
Software is installed in obscure ways.
A decade ago, software installations involved little more than creating a directory and copying some files. Not until DOS 6 were operating system files even compressed. Software no longer comes from trusted sources. A decade ago, users were counseled to avoid viruses by only installing software from trusted sources, and counseled to not download software from untrusted sources. And at 2400 baud, users spent some time judging the potential value of software before downloading.