Can you please provide some more information on this like;
What is the error message that you receive while the files are transmitted.
What is the type of internet connection the computer uses (dsl/wireless)
Open security center and let me know whether it states Am i protected YES or Your computer is secure ?
Messenger Option File Transfer click on the check box Scan files virus using - Click browes and look mcagent.exe on the McAfee folder in program files. Maybe this its your problem