Tittlemouse, RSI is a serious condition, and you must get proper medical help or you risk doing lasting damage to yourself. We had a lot of it at work some years back, and I will never forget the sight of a young woman in her 20s using her whole body to push open a door because her hands were not strong enough and the pushing caused her too much pain. Careers and lives were ruined by it.
You should really stop using the PC altogether for at least a while until the symptoms ease, and then take a long hard look at your setup. Are you sitting at the right height, is your monitor at the right height? Think about using a pen-type input rather than the mouse, make sure you take plenty of breaks ... I can't stress enough how serious this can be.
A quick google search turned up this: click here
it looks pretty good on its advice.
Without fail, and without delay, get medical help. If your GP doesn't take it seriously, tell him/her that you DO take it seriously and you want to be referred to a specialist.
Sorry to sound alarmist, but I've seen too many people seriously damaged by RSI.