This seems to work fine for me. I dont quite understand why you are making a copy of y:\medicomm.xls to y:\medicomm-@year-@monthno-@mdayno.xls and then right afterwards saving the edited file over it, so I just commented it out. The /y switch is also not known to me in kix. Also got rid of the console by adding a few $nul= lines. But anyway give this a shot.

 Code:
$app = CreateObject("Excel.Application")
$app.visible = 0

$file = "Y:\medicomm.xls"
$wbs=$app.Workbooks.Open($file)

;$app.Range("A1:H1").insert

$app.Range("A1").Entirecolumn.NumberFormat = "mm/dd/yy"
$app.Range("I1").Entirecolumn.NumberFormat = "mm/dd/yy"
$app.Range("Q1").Entirecolumn.NumberFormat = "mm/dd/yy"
$app.Range("T1:W1").Entirecolumn.NumberFormat = "$#,##0.00"

$nul = $app.Range("X1:Y1").EntireColumn.Delete
$nul = $app.Range("A1:W1").EntireColumn.Autofit

$filename = "Y:\medicomm-"+@year+"-"+@monthno+"-"+@mdayno+".xls"

$nul = $app.ActiveWorkbook.SaveAs($filename)

$nul = $wbs.Close
$wbs = 0
$app = 0