Thanks for all the suggestions guys. Everything appears to be working great - my PC even seems a bit faster.

Just in case anyone else gets stuck with this problem, below are the steps I took -

  1. Performed a system restore
  2. Downloaded Malwarebytes (thanks to Saintly and MV75) which picked up 12 infections
  3. Downloaded updated version of Avast 5.0 and ran a full system scan
  4. Created a new system restore point and deleted all previous restore points


It feels like a new PC