If you've been running the BETA of Service Pack 2 that was released last year, then you'll need to uninstall that before installing the official service pack. Plus, you'll also need to have Service Pack 1 installed first.
Although the Service Pack hasn't made it to Windows Update yet, you can now grab the official downloads from Microsoft's Download Center. The installer includes Service Pack 2 for both Windows Vista and Windows Server 2008, resulting in a 348.3MB file for 32-bit version - and a 577.4MB file for 64-bit version.
Despite the massive file size, however, there's not much to get excited about. The update mainly includes all of the bits and bobs that have been released since Service Pack 1, although this doesn't include Internet Explorer 8.
Microsoft says that the Service Pack will also bring you native writing to Blu-Ray discs, improved Wi-Fi performance when resuming from Sleep mode and support for Bluetooth 2.1.
The Service Pack will also include the ID and vendor strings for VIA's 64-bit CPUs. It's not exactly a must-have download, but Microsoft says that, "while most updates contained in SP2 are available as individual downloads, the roll-up convenience of SP2 is a major benefit for administrators."
If you're thinking of updating to the new service pack, then you may need to remove a few language packs before you start if you've installed them already.
The new service pack is currently only available in English, Japanese, Spanish, German and French. In fact, if you've installed any other language packs, then you'll need to remove them in order to install Service Pack 2 at the moment. However, Microsoft says that it has plans for an ‘additional 31 languages together in a second wave sometime after the first release.'
In the meantime, the link for the all-language standalone installer is currently broken.