Newer version of installer / service pack?

Well if the OP doesn't need or want support, and the computer is firewalled to FCUK, then that's not relevant.
The point is that his installer won't run. Whoever wrote the installer software will almost certainly tell him to upgrade to a newer OS . No-one tests their software on obsolete (i.e. unsupported) operating systems, Might work, might not...

Mrs Slocombe

The installation of software is ultimately the addition of files and registry entries. Thus you may be able to create an MSI by installing the application on another OS and capturing the installation.
Another way may be to temporarily increase the value of whatever the installer is checking in the registry in order to fool it. But as you say, might work, might not.