重新下了一次，结果。。。。

steam上买的怎么可能还要序列号？买来后直接就可以下载安装了啊

你在steam上买这些东西的话，他会给你发邮件，里面有序列号.
去自己的邮箱找找看吧.

在游戏库里的左边栏中找到那个游戏（名字为亮白色也就是下载下来过），右键查看

Right-click on the game in the Library tab and select View Game CD Key. This option is not available for all Steam games and a CD Key is not required for all games.
Verify your GCF files. Follow the directions in the Verify Integrity of Game Cache section of the Verify Game Cache Files (GCF) article.
For Windows Vista users, follow the directions under Try Running Steam as Administrator in the Windows Vista Troubleshooting article. Then, verify your GCF files again and test the issue.
For Windows XP users, on rare occasions you may need to properly set registry permissions:
Exit Steam.
Go to the "Start" menu, select "Run", type "regedit" and click "OK".
Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\VALVE
Right click and select "Permissions"
Check the boxes for "Admin" and "System" to "Full control" and "Read".
Restart Steam and then verify your GCF files again.

按照上面的翻译，应该是点那个验证游戏缓存的完整性。先试试吧，如果还不行，估计就得联系客服了

好吧，LZ遇到了跟我昨天一样的问题
去你steam的文件夹找到userdata，然后进去打开找到某9开头的数字文件夹，之后再打开其中的config的文件夹，删除里面的“localconfig.vdf”
最后重启steam看看能不能解决问题