finally upgraded from xp sp1 to sp2. after all the updates, i cannot install any software that is not microsoft's or comes as a self extracting .exe (can't install cc cleaner, adobe, etc). i can install office, defender, etc.
i first get a security warning that the publisher cannot be verified and then an error such as setup.exe has encountered a problem and needs to close.
What do your event logs show as happening? Clear those out and save a copy for reference later. Run your installs again and see what errors/warnings are listed in there.
You can also look for the log files created by the MSI file... which you can force by using a command line to launch the MSI.
Copy and paste this in to your RUN line and edit the obvious
/v will log verbose, which is every action taken by the installer I believe... can't remember
Somewhere at the bottom of that logfile, you are going to see a status of the installation. Those logfiles are a pain to read, but they do contain some good info in them.