Yes, the laptop is likely to be overheating and the thermal protection system is slowing it down to protect the hardware. Get the fan replaced as soon as possible and try not to use the laptop in the meantime.
The make is a compaq presario around 6 years old and running xp home.
woodchip..I think it may be the fan becasue the laptop feels quite hot underneath, I didn't realise it might be the harddrive, what could be wrong with that.
I would say that your fan is ok but your laptop needs to be serviced. This involves stripping down the machine, cleaning the vents & fan. The processor, graphics chip & heatsink is also cleaned & new thermal paste added. This operation will most certainly fix your overheating laptop.
Thank you all for your help. Just one last thing, if this repair is too expensive then my friend who is not computer savvy will buy another laptop, but she wants to keep XP home on the new one as she is used to it. Can we buy a new laptop with XP on it?.....
After six years of use it is very likely you have a lot of dust/fluff around the fan an cooling ducts/fins as spiderlegz indicates. You really have only two options, to do it yourself or get it serviced.
If you want to have a go yourself, there is a video which may help and a simple suggestion here.
Personally I would be tempted to do the minimum of disassembly and use a vacuum cleaner to remove as much dust/fluff as possible. Obviously the laptop must be switched off and care taken when using the vacuum not to do any damage.