WARNING: Uninstall any beta applications before installing any service pack as they may be locked in place in the process which would mean a format and reinstallation of Windows should you need to get rid of them.
Vista SP1, if not already onboard, should be installed first following the same principles laid out below. The standalone download for that is HERE (x86) and for x64 - HERE. (Change Country at bottom left if necessary)
Vista SP2 Standalone is HERE (x86) and for x64 - HERE. (Change Country at bottom left if necessary)
Temporarily disable Anti-Virus (Real-Time Scanning) and Personal Firewall:
Open SecurityCenter by either right-clicking or double-clicking the taskbar icon
Click Virus & Spyware Protection then Real-Time Scanning and switch it off and set to 'Never'.
Then go to Navigation (top right), then General Settings and Alerts and click Access protection to expand it, turn it off by unchecking it and click Apply.
Click Web and Email Protection
Click turn Off and tell it to 'Never' turn back on (until you tell it to)
(Your computer may automatically reboot several times during the installation of SP2).
Information about Service Pack 2 for Windows Vista