With Windows Server 2003 many people found that Analysis Services (SSAS) was slow to get going sometimes. So a preallocate setting was introduced that allows you to specify the initial memory setting that SSAS will get when it starts. This setting is a percentage of the total memory, and you want to be sure this is set below your Low and Total memory limits. If not, the server will immediately start to clear things out of cache, or try to.
This is another entry in the XML configuration file for SSAS. It is set with the memory limits as follows:
Note that this doesn't make a different with Windows Server 2008. The OS is more efficient in allocating memory to SSAS immediately and using small allocations. There's a great blog post at MSDN that talks about this: Analysis Services Preallocate Memory Setting – Insight.