I worked on a system that had this problem. I found
through searching the Net that some process was blocking
them. (sorry don't recall which one) A way around it was to
temporarily rename the .exe file (e.g. SpybotSD.exe >
New-SpybotSD.exe) Once that was done the program would run.
Once the problems were cleaned up, make sure you go back
and rename them back to the original names. The programs
that I saw affected by this were Spybot, Ad-Aware and
Norton AV, but I'm sure there are others. HiJackThis still
worked.
Hope this helps.
Gary