VBA Slow To Assign A Null String To A Cell - On One Computer Only

I have a VBA program which is running very slowly on one of my computers, but none of the others.

Here's a piece of code which illustrates the problem, which is that assigning a null string to a cell is very slow.

Option Explicit
Public Sub test()
Dim objSh As Worksheet
Dim I As Integer

Set objSh = Worksheets("Sheet1")
MsgBox "test1"
For I = 1 To 1000
objSh.Cells(1, 1) = ""
Next
MsgBox "test2"
For I = 1 To 1000
objSh.Cells(1, 1) = " "
Next
MsgBox "done"
End Sub

If I change this to assign a space instead it is very fast (about 1,000 times faster).

This slow machine is not normally slow; an ACER Aspire 9410Z with 2GB RAM, running Vista Ultimate.

Other machines run this code OK on a mix of Windows XP, 2000 and Vista Business. with Excel 2003, 2000 and 2007.

Ultimate seems to be the only unique factor of the machine where it runs slowly.