Does it happen when you emulate mouse buttons with the keyboard? LCOMMAND + LALT = LMB, RCOMMAND + RALT = RMB.

In your other thread you mentioned some problems with a keyboard, disconnect it and try without.

Open Exchange and look for programs which can change mouse/keyboard behaviour. For example MMKeyboard.

Some programs can also start from WBStartup, or S:User-startup.

Delete settings files (ENVARC:sys/icontrol.conf, ENVARC:sys/poseidon.prefs) and reboot, backup these first if needed.