There is a great post on your problem exactly here - http://weblogs.asp.net/omarzabir/archive/2007/10/19/a-significant-part-of-sql-server-process-memory-has-been-paged-out-this-may-result-in-performance-degradation.aspx

Please reply and comment as to whether it was useful and fit your situation. There are several links there to MS articles as well so, be sure to check them out as part of your TS.