Seems that something interfere with the installation of SP2. Please download and run the
System Update Readiness Tool for x86 OS. This one is just for Vista x86. This tool will try to fix and prepare your computer to update. After that reboot the computer and immediately install the Service Pack2 which you previously downloaded. I guess you have the correct version of SP2 for your 32bit system.
Edit: Have you ever installed prerelease version Of SP2?