Hello, first off let me say that I love RCT3 and have only recently started playing since 2012. I recently have had some technical issues with my computer and had to factory reset. I of course backed up my files including the RCT folders.
After re-installing all my essential files & programs and windows updates, the first game I installed is RCT3 which I purchased through Wild Tangent via HP Games. Game is compatible.
OS: Win7 x64 - Presario g7-1310 - 6GB RAM
I ran the game in XP Service Pack 3 compatibility mode (and tried Service pack 2 mode also) with run as administered on.
So here's the issue: I know the saved games go to C:\Users\Sam...\Documents\RCT3 along with other saved items. What I tried doing is renaming the RCT3 folder to NEW RCT3, and then using my old RCT3 folder for the game to use in the hopes that I wouldn't have to start building parks from scratch again. Unfortunately, the game starts out OK then crashes when I attempt to make changes to my park. I investigated further and found that the PICS Rule File is the issue. I have made it to Tycoon level on all boards up to the last Soaked board in which Im at Entrepreneur.
I also attempted to bypass the PICS Rule File by starting a new game (which creates a new PICS Rule File & folder- Campaign 0000) and then opening my old parks (by transferring the scenario folder into the campaign folder). This works, however I am unable to play the unlocked boards (even though I've unlocked them all before). The first 3 unlocked boards read my old parks and I'm able to play them, however, after saving them the board is still locked (even though the park reached Tycoon level), which in turn does not allow me to move on to the 4th board and further.

Does anyone have any suggestions? Technical expertise appreciated!

It sounds to me like you could have done with a backup of some of your RCT3 registry entries. Sometimes an application will enter details, particularly unlock data or registration details, directly to the registry. It seems you lost the unlock you were relying upon when your old registry was deleted with the factory reset.

While some of the unlock data has been saved with your first three unlocked boards, when you open them they in turn are attempting to use a registry setting that is no longer in place. If you haven't backed up your RCT3 registry entries, in order to access the fourth board and further I’m afraid it appears you’ll need to play those three boards again on your newly reset system.