GAME CRASHES AFTER EVERY MATCH AND FREQUENT FREEZES DURING GAME!!!!

So I recently bought a brand new dell desktop tower and literally the first thing I did was download and install LoL. I did have some trouble installing where It got stuck at 99% but I managed to fix that. As I was playing I kept getting random freezes for about 3-5 seconds then the game would come back on. (no big deal) but then every time I finished a match and clicked on the continue(victory/defeat) it would just close the client and I couldn't open it from clicking on the icon at the bottom. I have to close out the client and log in all over again. PLEASE HELP