" I have various software defences including Norton "
Please tell exactly what defences you have installed.
The RunAs program runs a program as a different user, requiring the correct domain, user name, and password. The user will be prompted for any required value that was not entered on the command line.
But can be disguised as Malware etc.
Run your Anti's (updated) and in safemode.
Mbam click here download/update and boot into safemode and run.