My badness I made a mistake when I said Vista SP2 from M$ does not have any prerequisites. In reality there are two prerequisites for installing SP2:
1. Windows Vista must have Service Pack 1 installed.
2. Users updating through Windows Update/WSUS must install an update to the servicing stack (described in knowledge base article 955430) that handles installation and removal of software updates, language packs, and optional Windows features. This update is necessary to successfully install and uninstall the service pack;
LINK to read the rest of M$ Notable Changes in Vista SP2 posted at M$ Technet.
Sorry about that :-)