#assumes that the files exists
$WorkbookPath = "c:\path\to\file\test.xlsx"

#patterns that you are trying to pull out of the txt doc
$Patterns = "User","Computer","Risk Name"
$VirusAlert = get-content "c:\path\to\file\virusalert.txt"

#the name of the sheet inside of the workbook
$sheetName = "Sheet1"

#open excel file and select sheet
$objExcel = New-Object -ComObject Excel.Application
$WorkBook = $objExcel.Workbooks.Open($WorkbookPath)
$WorkSheet = $WorkBook.sheets.item($sheetName)

#grabs the used range from the workbook
$usedRange = $WorkSheet.usedRange

#grabs the last cell in the used range
$lastCell = $usedRange.SpecialCells(11)

#new row is the next down
$newRow = $lastCell.row + 1

#Loop through all data in txt file you care about
foreach ($Pattern in $Patterns){

#pull the line you want from the txt file
$AlertValue = $VirusAlert | Select-string -pattern $Pattern -list -quiet

#split off the labels, keep only the data you wanted
#this will break easily if the format isn't consistent
#or if you were to have an extra space for ex. computer name
#would only caputer "name"
#"User:  Me" --> "Me"
$AlertValue = ($AlertValue -split " ")[-1]

#Find the column with that header
#also breaks easily as this must match exactly and has to be unique
$col = ($Worksheet.Columns.Find($pattern))

#insert data into the new row in the col number we just found
$WorkSheet.cells.item($newRow, $col.Column).value = $AlertValue
}#foreach pattern

#save and quit
$WorkBook.Save()
$objExcel.Quit()