DESTINATION is empty. Expanding on Keith's suggestion:
foreach ($computer in $computerlist) {
if((Test-Connection -Cn $computer -BufferSize 16 -Count 1 -ea 0 -quiet))
{
$OS = Get-WmiObject -Computer $computer -Class Win32_OperatingSystem
if($OS.caption -like '*Windows 7*'){
$DESTINATION = $DESTINATION7
}
if($OS.caption -like '*Windows XP*'){
$DESTINATION = $DESTINATIONXP
}
}
}
This could avoid the error you're getting also. empty $DESTINATION
.