casting a GUID from a string?

Hi everyone-

I'm using GUID's stored as strings in Excel to manage a really big project, which can't all fit in a single definition.  Basically, I'm using Excel to retain information about each object (tree info, relationships to certain other objects, etc.) after it's been baked, so that our team can hand tool some of the results, delete certain objects, etc.  I'm using the doc.objects.find(guid) function right now - which works fine when I feed a string into the VB component and set the input as a GUID, but am having a hard time casting my strings from Excel into the GUID directly in the VB component.  Hopefully it's easy to do and I can whack my palm on my face, as often I do.  Here's my script...I get the "specified cast is not valid" error at: Dim obj As Guid = xlSheet.Range(strGUIDColumn & I).Value.

If activate = True Then

      Dim xlApp, xlSheet As Object      

xlApp = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application")
Dim strSheet As String = "MEM_6"      

Dim strGUIDColumn As String = "C"      

Dim strDeleteColumn As String = "F"

Dim intCheck As Int16 = xlApp.Worksheets("META").Range("B4").Value

Dim I As Int16

xlSheet = xlApp.Worksheets(strSheet)

For I = 2 To intCheck + 1

Dim obj As Guid = xlSheet.Range(strGUIDColumn & I).Value <- returns my casting error

If doc.Objects.Find(obj) Is Nothing Then

xlSheet.Range(strDeleteColumn & I).Value = "X"        

End If

Next

End If

thanks!