.bak is a backup file of your park. If you could rename the file and remove the .bak part so it only says "parknamehere.dat" and then try and load it into rct3 again.

Make a backup up both files just in case because the backup doesn't always work.

I renamed them and still, "unable to rad file -it may have been corupted." Downloading the custom scenery wouldn't have cause it would it? Thank you all for your help! If you have any other suggestion, please let me know. Thanks!
-B

Yes, custom scenery may have a problem. I would reccommend reinstalling any custom scenery you have. Without it, it would render the park useless throwing errors right and left. So if you could get it to work with the custom scenery, I would either keep it that way or delete the custom scenery from the park, save it, then uninstall the custom scenery from rct3.

well, I tried all the suggestions and it still doesn't work. So I guess I'll just have to start over. If anybody wants to try getting it to work Ill send it to ya. I guess just PM me your email...

FYI: There are no virus! At least Norton says so.
Thanks!