I've been using TP for a while with no problems but the other day I started not being able to run it other than as a full-screen window. When I double click the title bar or click the "restore down" button, TP collapses (kind of Mac-style) to just the title bar, no content window visible.
Please try deleting the file config.xml, which you will find at the following location, and then try starting up TextPad. Exit from TextPad before deleting the file.
C:\Documents and Settings\User name\Application Data\Helios\TextPad\5.0