could be a few trhings!
overheating in portables is or can be a problem. Does the vent feel as though the exiting air is warm? (if so how much a little warmth OK a lot not good). Try putting it in a cooler place and ensure all vents are free of dust and the machine has a clear airflow all round and under it.
If not heating it could be an electrical fault, failing memory or mother board. memory can be tested or swapped with another batch to see if that solves it. If it is motherboard or other electronic components on the Moboardthen you need expert electrical diagnostic help.
Thanks. It doesn't feel too warm.When it's booted up and idle it seems ok, its when you start to do something the problems occur. I cant do anything like check the task manager to see where the problem is. It was sat unused for a couple of months, which is why I thought it was the battery.
there is another small battery on the motherboard that keeps the cmos, and date and other things alive even when the main battery is removed. it is possible this battery has died (if so it will revert to some default settings that may give the symptoms your are having..... leaving it on charge should top up this battery but you may have to reset the cmos settings (or provide a new small battery)
Also there is usually a small hole in most portbles to do a factory reset. it is worth trying inserting an end of a paper clip or similiar and trying that to see if it works.