This could be problematical. I do not even know if the windows.old contains your data. This reference I looked at http://support.microsoft.com/kb/932912 does not say for sure if the application data folder is included. Others do.

So the first thing I think is to make windows show hidden files and folders. Then see if the application data folder is in there somewhere and if it contains a Thunderbird folder.f

The application data appears to be there under /Documents and Settings/Administrator/Application Data/Thunderbird and the Profiles folder contains 32 folders and nearly 4 GB.

Well we stand a chance.

in the profiles folder there will be another folder (most likely named XXXXX.default where XXXXX is a random string.) which is you old profile folder.

Download and install Thunderbird. DO NOT configure accounts, opt to do it latter. On the help menu open the troubleshooting information click on the show folder button. Close Thunderbird.

A windows explorer window will have opened showing the current profile folder. Again with the random string (a new one) and the default

In the explorer windows showing your old profile click on the edit menu and select all and then copy.

In the profile window that Thunderbird opened click paste

Start Thunderbird and hopefully everything is back to normal.

Ok, all fixed now, thanks very much Matt. For general information the trick is that in Windows 8.1 Thunderbird stores Profiles in 2 places - C:/Users/User's Name/AppData/Roaming/Thunderbird and also in C:/Users/User's Name/AppData/Local/Thunderbird. The one you need to copy the files to, as described by Matt, is the Profile in the Roaming directory. And even if you have inadvertently configured a new account (as I had done) the above instructions work. Just be sure to click overwrite existing files at the end of the file transfer process.