Mavericks messed up my macbook

Hello Everyone..!!

I use a late 2009 unibody white macbook which was initilally shipped with Snow Leopard. Ever since I downloaded Mavericks, my whole system has just turned into a big mess..!! Let me highlight some of the major issues that I'm currently facing :

• App's don't launch - App Store / Software Update / Terminal / iMessage / Photo Booth etc etc
• App's that crash often - Transmission / Safari / Finder

My system's gone slightly laggy but again that's something I totally understand considering my system runs a dual core processor. My major concern is the applications that don't even open.

What do i do know..??

Gaggy.