I have run into two issues when attempting to install the trail versoin of PrE 9. One of the issues is severe enough that it required a system restore.
1) PrE cannot complete installation because MSXML 6 fails to install.
2) PrE rolls back the installation and this causes a corruption of all installed installations.
...1) the issue seems to be for those of us who already have MSXML 6 service pack 2 installed. PrE should not be attempting to downgrade MSXML from service pack 2 -- which it apparently is trying to do. MSXML is system protected in XP SP3 and actually can't even be un-installed. MSXML was installed as part of Microsoft's Visual Studio application. I'm not sure when the service pack was installed but you can see more by looking into Microsoft KB954459.
...2) When the rollback begins, I start getting dialogs asking me to 'configure' an application. The dialogs disappear and reappear multiple times. This behavior continues when the rollback is completed. You cannot shut down the computer with these dialogs on the screen as the system prevents a shutdown. I tried to cancel after watching the screen for 30 minutes. Finally, I started to kill applications. I was able to reboot.
When the system came back up, the configuration dialog appeard every time I would run a new application. For most of the applications, the configuration would complete and the dialog would not appear again when the program is re-launched. However, several applications were corrupted (probably because I was killing the tasks to configure them to get my system to reboot).
I made a backup of my complete system prior to installing PrE. I was able to restore to a state prior to installing PrE without issue (thank you Acronis!)
I realize there are millions of different system configurations. You may want to take some extra precautions if you do have KB954459 installed on your system!
I'm looking forward to a resolution of this issue and will provide any additional information as required.
In the end, I would like to install a 'real' version on my video editing workstation which is running Vista and may not have this issue. However, I would really like to take it for a test drive on my other system first as it is far easier to 'protoect' a 80 GB hard drive than a system with multiple (and larger) drives.