With Toad closed, go to the Toad for Oracle application data directory:
C:\Users\<personal folder>\AppData\Roaming\Quest Software\Toad for Oracle\<version>\User Files (Win 7 or Vista)
or
C:\Documents and Settings\<personal folder>\Application Data\Quest Software\Toad for Oracle\<version>\User Files (XP)
rename/reset the TOAD_SESSBROWFILTERS.INI file.