Latest Norton/Symantec antivirus program appears to eat the executable files from RADMIN. The solution is to exclude the directory where the RADMIN program is located from both the Symantec full scan list and the Auto-Protect (real-time scan).
There was also a reference to the fact that Microsoft forbids the use of anything other than Terminal Services for remote access, in the license for Windows. I hope this is only an urban legend.
More at the Famatech web site.
Update 10/24 Still doing it!
Here is a fix from Symantec.