Clicking doesn't work anymore

I have a 15" MacBook Pro dating from mid 2009. Since the update to Mavericks clicking doesn’t work anymore, at least not properly. Sometimes it doesn’t work at all, sometimes I have to use right-click instead of left-click. Repairing access rights (hope this is the correct translation from German "Zugriffsrechte") helped only partially. Any incompatabilities known in this respect? Or any other suggestions?

Thanks!

t.