Your server is hitting the limit it’s operating system’s paged pool memory. This is more commonly seen when large databases are opened by multiple users and the cache is not released properly, or third-party applications like bacup agents, virus scanners etc are used. Restarting the server clears the memory pool and lets you start fresh again, until it fills up to it’s limit once more.
View IBM’s info on this at http://www-1.ibm.com/support/docview.wss?rs=203&q=179781&uid=swg21093511&loc=en_US&cs=utf-8&lang=en Though it starts off talking about NT4.0, it mentions Server 2003 at the bottom, and also includes references to Microsoft support articles.
It would be interesting to see if these messages were appearing at a common time.
Also, depending on how heavily used your server is, it may be a good thing to schedule a regular reboot anyway (once a month maybe).