Sims 3 won't run right?

Okay, I was trying to clear a little bit of space on my computer a couple months ago and I got rid of some programs I didn't recognize, thinking they weren't important. Apparently one of the programs was and now my Sims 3 games don't work correctly (this is a HUGE problem for me considering all I ever do is play the Sims...). The problems were happening all the time and they include sometimes not starting after I hit play on the launcher, problems updating the game, and my main problem, the mood is glitched. Once I start a family, it works correctly but gradually after a few days even without a single moodlet, it says my sim is miserable. I took all my sims games off of my computer about a month ago and put them all back on last night. Every time I open the launcher, an alert message comes up with the heading "MessageDlg.exe - System Error" and it says "The program can't start because QtCore4.dll is missing from your computer. Try reinstalling the program to fix this problem." Anybody know what that is and how I can put it back on my computer??