Vista - Stopped working after updating XP

I've used the script below for the last 2 months without problems and then
earlier today I thought I might update my Windows XP from service pack 2 to
service pack 3. Executing the script now throws up the error "Type Mismatch:
Left" and "Type Mismatch: Top". I think (I might be wrong) it's because the
parameters intHorizontal and inVertical somehow remain null.

Does anybody know why ? Thanks for your help.

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'Get screen width and height
Set objWMIService = GetObject("Winmgmts:\\.\root\cimv2")
Set colItems = objWMIService.ExecQuery("Select * From Win32_DesktopMonitor
where DeviceID = 'DesktopMonitor1'",,0)
For Each objItem in colItems
intHorizontal = objItem.ScreenWidth
intVertical = objItem.ScreenHeight
Next

'Display progress indicator
Set objExplorer = CreateObject ("InternetExplorer.Application")
With objExplorer
.Navigate "about:blank"
.ToolBar = 0
.AddressBar = 0
.MenuBar = 0
.StatusBar = 0
.Left = (intHorizontal - 450) / 2
.Top = (intVertical - 250) / 2
.Width = 450
.Height = 250
.Visible = 1
.Document.Body.Style.Cursor = "wait"
.Document.Title = "Progress Indicator"
End With