What is the correct syntax for this If statement?

Experts,

       Below I have code that will install another script based on the OS. However the last else statement is printing every time the script is run for either OS. How do I get the program to print Computer is unknown only if XP or Win2k3 is not found?

Thanks
Missymadi

if(get-PSSnapin Quest.ActiveRoles.ADManagement) {write-host Quest Tools are installed}
 else {Write-Host "No Quest Tools are installed"}
 Add-PSSnapin Quest.ActiveRoles.ADManagement
 Write-Host "Quest Active Roles for Active Directory is now installed on your system." 

$strComputer = Read-Host "Enter Computer Name"
$OS = gwmi win32_OperatingSystem -Comp $strComputer

# if statement to run code for Windows XP and Windows 2003 Server.
write-host "Computer Name: " $strComputer
if ($OS.Version -eq "5.1.2600") {
write-host "OS Version: Windows XP"
.\PromptReset.ps1
}
else
{
write-host "OS Version: Windows 2003"
.\PromptReset.ps1
}
}

#Unknown OS

else {write-host "The OS for: $strComputer is unknown"}