Excel 2010 InteropServices fail

Hi everyone-

So...due to increasing buginess and crashing issues, I had to reinstall Windows 7, which meant losing all the software that came with the disk image I got as a student, including MS Office 2007.  So I went out and bought Office 2010, and all my Excel interopservices vb.net scripts are now failing.  They had all worked fine before...now when I execute they give the following error:

error: Invalid class string (Exception from HRESULT: 0x800401F3 (CO_E_CLASSSTRING)) (line: 0)

Here's the boiled down script:

If activate = True Then

Dim oldCI As System.Globalization.CultureInfo = system.Threading.Thread.CurrentThread.CurrentCulture

System.Threading.Thread.CurrentThread.CurrentCulture = New System.Globalization.CultureInfo("en-US")

Dim xlApp As Object

xlApp = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application")

Dim xlSheet As Object

xlSheet = xlApp.ActiveSheet

xlSheet.Range("A1").Value = "test"

End If

The failure happens with the addition of the InteropServices line.  I'm mid-project right now, and it's pretty critical that I regain the ability to go back and forth from GH to Excel in the scripting environment...does anyone know if I need to deactivate any security options in Windows, make sure any dll's are properly installed...anything? I've tried all of the other Excel components that various users here have already been generous enough to share (my scripts are all based on them).  I can't seem to find any documentation (that I understand) about this particular error message, or how to fix it.  Really could use a little help!

thanks in advance,

Dave