I've struggled for a day and a half to get the 5.0.3 update to install (I don't see where the 5.5 version is worth $179 to me).
What happened - repeatedly - is that I'd select "Update" in 5.0.0 and it would download for quite some time, install for quite some time, then give the cryptic message, "Installation Failed." Not terribly helpful.
I finally got it to install successfully, and wanted to share the "how." Here's what worked for me:
Uninstall Premiere Pro 5.0.0.
Download and run the CS Cleaner Tool. (This is the secret weapon!)
I don't know if it was necessary, but I used the Cleaner to remove everything that had anything to do with CS 5.
Then I ran the installer I had downloaded yesterday (PremierePro_5_LS7.7z). It's a huge download - nearly 1.5 GB. Once it extracts everything, kind of buried in the Adobe CS5 folder is the setup.exe file you need to run to do the installation. Make sure Premiere Pro is closed (and use the Task Manager if you have to kill it - sometimes mine keeps running after I've closed it).
It will take awhile to install, but that's basically it. I now have 5.0.3, and all the other components are current, too.
I hope this helps someone else avoid the struggle I had.