A constant bad checksum is down to only a few things...
In most cases Game files youve downloaded are corrupt, can be caused by an intermittent internet connection, a fault in your ram, application interferance with whats held in your ram causing corruption when the file is moved from temp to the location you wanted it, or hard disk corruption.
To be safe you want 10gb+ free on your drive, depending on the size of your hdd and your recycle bin settings, windows will reserve a portion of your drive you dont see happen.
Closing all applications that are running in the system tray is always a good thing to try, this rules out many apps interfering with whats in memory as operations take place.
Depending on the quality of your net connection, the corruption may be occuring in the download process. Other than trying a full reinstall a few times, trying installing to different directory, and doing a tracert to the download servers, which i cant talk you through yet as i dont know the ip at this time, there isnt much else we can try here atm to test that.
Try running an error test on your ram to see if its causing the corruption. There are a lot of free memory diagnostic tools, if no errors are found then this can be ruled out as an issue. Try
http://www.memtest.org/
Do a thorough scandisk, and then a full defrag. The corruption could be due to the game trying to install on a bad sector, a thorough scandisk will stop the application using this sector. Then the defrag will make sure all indexing on the drive is in order which could also be an issue.