My laptop and ancient desktop both run XP successfully and I don't have a problem with YouTube or any other programme.
Taking the cover off the fan is easy - use compressed air (available at maplins and pc outlets) to whoosh the dust and fluff away. Or if your vacuum cleaner has a low suction setting, you can use that with care.
It will make a big difference, I'm sure.
If you want to fit more RAM this is also straightforward, but make sure you buy the right sort.
Also, make sure the air can circulate round your laptop - don't sit it on a soft surface which will block the vents.