This is one of those things that's hard to put your finger on. Sometimes Premiere may start behaving oddly. Unexplained sluggishness is the most likely symptom. A possible fix is to create a new Preferences file. The quick keyboard shortcut to do that is to press Ctrl+Shift (Windows or Mac) before double-clicking the Premiere program startup icon on the desktop.
That will bring up the initial Premiere 6.5 opening screen, asking you whether you want to switch to A/B or single-track editing.
One caveat and one surprise:
You can also rename or remove the Preferences file. In Windows you should find it at C:\Documents and Settings\[username]\Application Data\Adobe\Premiere. This is a hidden file folder, so you may not see it. If you search for it, make sure your search parameters include Search Hidden Files and Folders. On Macs it's normally in the Preferences folder. System Folder, Preferences for System 9.0 and User, UserName, Library, Preferences under Mac OS X.