Mr D - do heed spurlash's post - it is often an '"I hate M$" virus that does this blocking, and can be written to 'hide' from programmes like Malwarebytes when run outside SAFE mode and sometimes even in SAFE.. Are there any other symptoms like pop-ups, redirected websites, home pages etc?
Please tell us something about your AV programme - what is it, is it updated and does it offer a boot-time scan? Have you run MBAM in SAFE? You may need to run a boot-time scan to look through your system BEFORE it gets anywhere near to M$-land