I have kept me Plex Media Server on a Server 2003 box for years. Have installed update through 9.9.16 that says Windows Vista is min OS. Have never had a problem with update not installing. Went to install the latest 9.10 and 9.11 updates this morning and the same system OS requirement is listed (Vista) but now the install says that I need a newer version of the OS to install.
If you running WHS version 1, that is based on Server 2003 which as said above is no longer supported. You will need to upgrade to use the newest release.
Microsoft still support Server 2003 and WHS V1, for some using WHS you cannot simply upgrade the OS as these systems are headless. Upgrading the OS will typically mean junking the whole machine and buying a new one.