Dynamic LINQ query

I am in the process of creating a Windows application in VB (Visual Studio 2008) that uses LINQ. All my data is stored on an SQL database.
There is a main grid that I can populate quite easily but the problem comes when I try to populate the grid using some search criteria.
The problem is that I have multiple text boxes that the user can either enter search criteria or leave blank. There is no way of knowing which fields in the database I need to look at and compare until the user clicks 'Search'

e.g         Dim Claims = From Claim In myDatabase.Policies Join ClaimStatus In myDatabase.Status On _
                                   Claim.StatusID Equals ClaimStatus.ID Select _
                                   Claim.ID, Claim.SolsRef, Claim.PolicyNumber, ClaimStatus.Name  _
                                   Order By ID

This returns all rows of the database. I can search for a particular claimID using the following:

        Dim Claims = From Claim In Bwise.Policies Join ClaimStatus In Bwise.Status On _
                             Claim.StatusID Equals ClaimStatus.ID Select _
                             Claim.ID, Claim.SolsRef, Claim.PolicyNumber, ClaimStatus.Name Where ID = idVariable _
                             Order By ID _

but what if I want to search for something else like policy holders surname or a combination of more than one field? Do I have to define a new query for every possible comibination, I dont think so as this would be almost unmaintainable. Any help would be greatly appreciated.

Thank you in advance