I wrote a script to copy files to the "All Users" desktop or "Public Desktop"
However we have a mixed environment. Some people are using Windows XP and other people are using Windows 7.
$SOURCE = "I:\Path\To\Folder\*"
$DESTINATION7 = "c$\Users\Public\Desktop"
$DESTINATIONXP = "c$\Documents and Settings\All Users\Desktop"
$computerlist = Get-Content I:\Path\To\File\computer-list.csv
$results = @()
$filenotthere = @()
$filesremoved = @()
foreach ($computer in $computerlist) {
    if((Test-Connection -Cn $computer -BufferSize 16 -Count 1 -ea 0 -quiet))
    {   
        Write-Host "\\$computer\$DESTINATION\"      
        Copy-Item $SOURCE "\\$computer\$DESTINATION\" -Recurse -force        
    } else {
        $details = @{            
            Date             = get-date              
            ComputerName     = $Computer                 
            Destination      = $Destination 
        }                           
        $results += New-Object PSObject -Property $details
        $results | export-csv -Path I:\Path\To\logs\offline.txt -NoTypeInformation -Append
    }    
}
                EASIEST METHOD: Click the Windows Start button and type msinfo32 and press Enter. Click View > Remote Computer > Remote Computer on the Network. Type machine name and click OK.
There are two PowerShell cmdlets for extracting OS information, Get-WMIObject and Get-CimInstance.
DESTINATION is empty. Expanding on Keith's suggestion:
foreach ($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.
In your foreach loop through $computerlist you can grab the OS Caption for each computer by using WMI:
$OS = Get-WmiObject -Computer $computer -Class Win32_OperatingSystem 
Ant then check the $OS
if($OS.caption -like '*Windows 7*'){
    #Code here for Windows 7
}
#....
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