Try launching the client with "-rescan" enabled.
I made an easy launcher to do exactly this and a couple of other tools in my "BTC/LTC Newbie Tools for Windows" package:
https://bitcointalk.org/index.php?topic=113707.0The tools are very basic but designed to be simple to use. Most of the tools you just drop into the appropriate directory and double-click.
Instructions are in the included "README" files.