A custom User Data folder path apparently can be too long.
I found that when I added another folder level in the default installation path for User Data folder, this made the complete file path about 262 characters long, counting spaces. This was just over the long-filename limit for XP, and resulted in an error message every time I tried to access the drawings directory from either the start window or the file>open menu. Reinstallation with shorter folder folder names solved the problem. I was able to considerably shorten the default directory name from something like " TurboCAD Professional 18" to TCAD 18", and that allowed me to add my preferred directory into the path.
It is a little disconcerting that although you can specify a custom path at intallation, there is no warning about the default path already being within about three characters of reaching the character limit for XP. It seems like a long user name could easily trip this error.
I hope this finds its way to helping someone else.