Click Start > (My) Computer
Locate the hard drive Steam is installed on. By default, this is C:
Right-click this drive and select Properties.
Click the Tools tab.
In the Error-checking category, click Check now...
In the dialog that appears, check "Automatically fix file system errors" and "Scan for and attempt recovery of bad sectors."
Click Start.
If you are scanning the C: drive, you will be asked to schedule the disk check the next time you start Windows. Accept, restart your computer, and allow the disk check to start—do not press any keys to skip the disk check.
Allow the disk check to complete. This process may take up to several hours, depending on the size of your hard drive and the number of errors.
Once your hard drive has been checked, verify the integrity of the game cache
Please see Verify Game Cache Files (GCF) for instructions.
If you are still seeing this error, move your Steam installation.
We recommend testing on another hard drive, but you can also try installing Steam in another folder such as C:Steam
We also recommend redownloading the game instead of using your existing files—you can back up your steamapps folder in the meantime.
The reasons for doing so are multiple:
It ensures a fresh copy of the files are downloaded.
It will generally install the files to a new physical location on your hard drive, avoiding the physical location that the issues are occurring at.
If there is an issue with your Steam installation, or a bug with Steam, it will reset the state of your Steam installation, possibly preventing the issue from reoccurring.
Please see the article on Moving a Steam Installation and Games.