Had the same problem on a Packard Bell Easy One Silver over the past few months.
On this laptop, there is a small hatch on the base near where the air comes out. This can be removed with a jewellers screwdriver. I found that there was a lot of fluff and dust covering the fan, which I removed. The fan iteself seemed to be turning OK, but all this did not stop the problem.
I rang PC World, to see if they could do anything, and they refused to even discuss it with me unless it was under their extended warranty. They did refer me to a company in Phibsboro who would replace the fan and heat sink unit. (Sorry can't remember the name).
In the end, I kept using the laptop, and the problem cured itself, so I never got a price for it or anything. I reckon I wouldn't have got out of it for less than 200 euro, parts and labour inc. NOTE : I wouldn't have opened it myself if it was still in warranty, in case anyone else out there is in a mood to invalidate their warranty...