grimx133

It won't be the fan itself slowing down the computer, it'll be the heat that the fan is having a hard time getting rid of. Probably clogged with dust bunnies. You need good airflow to keep a computer running properly. I'd guess that your case has never been opened, you're going to have to do so and clean it out. Pick up some canned air at an office supply shop or computer shop. Take the cover off and blow out as much dust as you can. Pay special attention to the cpu heatsink and fan. This'll be a fan that's mounted directly on the motherboard. Of course, the case fans need to be clean as well, but cpu heating is most helped by having lots of air going through the fins of a heatsink. If the fins or the fans are clogged, this doesn't happen. At this time, don't take anything apart, aside from opening up the case to get at the motherboard and fans. If the cpu fan is taken off, you need to thoroughly clean the old thermal paste/pad, and reapply fresh. If it comes to that you'll need to get some paste.

Get it clean as you can, and see how it goes from there.