If you know what you need on a server system, its better to just check that a .dll (with the api's you need) is present on the server system
(Also, the current version of WinVer.nsh will also report XP64 as Win2003)
You can also try my custom version of WinVer.nsh, it supports checking if the OS is workstation or server, you can grab a copy @
http://nsis.pastebin.com/f7844ed1a