firstly, shut off your internet on that pc. next download this (
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the first download link is hijackthis. this will help you identify where the rat is hiding.
the second link is fileassasin. this is to help get rid of the rat if (and it probably will be) it is protected.
then run hijackthis. have a look through there and see if any thing on the list looks strange. also, post your log and ill have a look through it for you. also, have a look in taskmanager for any processes you dont recognize. again, post a screenshot and ill have a look.
then, you can use file assasin to delete the files we just identified.
both these programs are very easy to use, hence i have not posted detailed instructions.
post back if you have any questions, hope this helps