The presence of 08 initally suggests Server 2008 but I doubt that's correct. The server is relatively old and it currently has Server 2003 installed on it (not necesarrily the OS that it came installed with)
I'm guessing 2003 but I just wanted to ask to make sure. As usual in cases like this Google is no help because it just interprets my '08' as '2008' and returns those results, or if I wrap in quote marks I get lots of not-very-easy-to-make-sense-of websites.