As of May 2013, the latest service packs for Microsoft Windows includes Windows 7 SP1, Windows Vista SP2, Windows XP SP3, and Windows 2000 SP4. There is currently no major version update for Windows 8.
Please note that for most of you, the easiest way to install the latest Windows service pack or update is to run Windows Update.
Download Locations for Microsoft Windows Service Packs & Updates
|Operating System||Service Pack / Update||Size (MB)||Download|
 Beginning with Windows 8, Microsoft will release regular, major updates to Windows 8 in the form of Windows 8.1 (previously code-named "Windows Blue"), Windows 8.2, etc. Windows 8.1 is likely to be released in August, 2013. Service packs will not be released.
 Windows XP SP3 can only be installed if you already have Windows XP SP1a or Windows XP SP2 installed. If you do not have one or the other of those service packs installed, install SP1a, available here, before trying to install Windows XP SP3.
 Windows XP Professional is the only 64-bit version of Windows XP and the latest service pack released for the operating system is SP2.