In most cases simply launching the client with "-rescan" will fix this problem.
I have made a very simple launcher to launch your client with "-rescan" and a few other simple tools included in my "BTC/LTC Newbie Tools for Windows" package:
https://bitcointalk.org/index.php?topic=113707.0The tools in it are very basic but designed to help those who may not be as technically capable as others. Most of the tools are as simple as dropping the file into the appropriate folder and double-clicking.