Vista Business - Can't Start Up Remote Assistance

I have Vista Business running under an admin user account, and I can't
initiate a Remote Assistance session by going to "Help and Support". I click
on the link "Use Remote Assistance" and nothing happens. I have turned on RA
by going to System, Remote Setting, Allow Remote ....; checked the Firewall;
etc. When I try to start RA by running msra.exe, I get an error message
"Windows cannot find C:\Windows\System32\msra.exe, even though I can locate
this file in the System32 folder. I have run SFC /scannow and everything is
OK. I also have this problem with "On Screen Keyboard" (osk.exe, same
symptoms) but everything else in Vista appears to be working fine. Any
suggestions would be appreciated.