Scan won't give a refund but they should give you an RMA if your in your warranty period. Failing that contact the manufacturer directly and explain that scan have refused to even offer an RMA. In order to get an RMA in progress you will have to know what device is causing the problem.
If you suspect the graphics card then try an RMA on that but it might not fix the issue.
Another option is to find someone local to where you like that does a lot of pc hardware repairs, and have them rebuild it for you. It might be something small that you have missed on construction. They should also have a rig that they can test your hardware in.