Most of the time a simple can of compressed air sprayed along the card will do it. WHen you spray the fan area, hold the fan still so it won't spin as you don't want to hurt the ballberrings within the fan. If you absolutely need to go deeper you can use a bit of alcohol with a microfiber cloth for some of the tougher areas but you want to remove it from the case then and let it dry. You do NOT want to be doing the resistors, capacitors or anything like that as you may bend them. Air will do just fine.