I just purchased the CS4 upgrade (from PS CS2) for my Windows XP system & it runs fine. But when I download ACR 5.2 update & try to install it manually it doesn't appear to install. The Help/About/Plugins window shows ACR version 5.0 running. When I try to install the 5.2 update using the Help/Update function, I'm told to close Bridge, but Bridge isn't open, so I get into a loop of message/retry/message/retry etc.
You probably turned on Bridge "stealth mode(auto launch Bridge when login)" by accident. You can use Windows Task Manager, and check the Processor page to see if Bridge.exe is running. Also, if Bridge stealth mode is on, you should see a Br icon on the system tray.