Not a valid Win32 application error

I own a Dimension 4400 and recently after running a macafee virus scan something happened.  I can no longer use any of the programs on my computer including the internet.   For example when I try to use microsoft word, I get an error box that says, (null) is not a valid Win32 application.   And this is the error message I get for every program I try to use.

 

Help!!  I don't know even where to begin to try and fix it.  Any help, would be greatly appreciated.

 

Thanks