As found in the support:
QUOTE
Nexus 6.5 Corrupted Download (I/O Error)
Did you encounter an error while trying to install the Nexus TK client?
Step 1: Check your user privileges.
If you are using Windows XP or Windows 2000 Operating System you will need to make certain that you have Administrator privileges to install Nexus TK. Logging on with the Administrator account will allow you to install Nexus TK.
Step 2: Clear your temporary directory.
If you have the proper Administrator privileges, or are using Windows 98/ME, and are still receiving an I/O error during the installation then you may need to clear out your Temporary
directory.
Clear out Temporary Directory:
a) Click Start->Run.
Type the following command then press Enter:
explorer %TEMP%
c) Delete all of the files in that folder.
Step 3: Delete the corrupt files and download new ones.
The only way to fix a corrupt file download is to clear your Temporary Internet Files, delete the bad file and try again.
1. Clear your Temporary Internet Files:
a) Open up Internet Explorer.
Click Tools->Internet Options.
c) In the "Temporary Internet Files" area click the "Delete Files.." button.
d) Click OK to delete the temporary files.
2. Delete the Corrupt file.
3. Download new files from the download page.
If you're using Vista / 7: In order to have the correct administrator privileges you must run the file as an administrator, right click it, the option "run as administrator" should be there, if it isn't then you aren't an administrator of your machine and you'll need to ask someone else that has full access to the machine you're playing with.