Six years is doing good. However give us a make and model number, that always helps things along. Have you ever removed the side panel and blown out the dust bunnies. This is the first place to start. If the computer rests on the floor move it up, even a 6 inch block will work wonders as all dust eventually reaches floor level and you disturb it as you walk about. The fans then suck it in. If it is dusty in there buy a bottle of compressed air, remove the computer outdoors (you don't want to upset whoever cleans the house) and blow the dust out. BE SURE to hold a finger on the fan so it does not spin wildly and pay particular attention to the fan on the motherboard. This protects your processor and very likely it is overheating and shutting down to prevent damage. If you live in a VERY dusty environment and it is bad in there then also take a look between the plastic front panel and the metal behind. There is a 3/4 inch space that can get bad. You can also test for heat issues by removing the left side and use a small house fan to blow in on the components. This is not a fix but a temporary measure so you can decide if heat is the issue here. Let us know how that works for you

I agree with Raven here, I would not be taking a Celeron computer in for repair. That might well cost half the price of a new computer. You don't have a friend, neighbour or relative who might loan you their Windows XP CD? Explain to them that you do not need the license number as your license sticker on the side of the computer case is the one you would enter as you install XP. That sticker number is tied to the motherboard in your computer. (I suspect that if you have XP Home then you would need an XP Home CD) Recovery disks are bought at the time of the computer purchase. Or this is more often just a separate partition on your hard drive and the buyer should create a set of recovery disks. This includes all the 'free' extras the computer maker adds on. The XP CD is just that, a copy of Windows XP with nothing added As a side note you may want to suggest to Dad that he creates a set of recovery disks for his machine in case he runs into problems too at some future point.

I use Firefox daily and never had this problem, I would not quickly attribute it to Firefox. But make sure you have the latest updated version of Firefox! Try clearing the cache on Firefox which should be done regularly anyway. Try Right clicking on the back arrow, that will give you a drop down list of the previous pages you visited, click here on the first selection which should be exactly the same as just hitting the back button once. If this works I think you might have a problem with the actual back button. Do you have any problems with just using the cursor and using it to click on the back arrow on Firefox? I don't think I have ever used the back button on my pc keyboard to go back to the previous page, I strictly use the mouse cursor, my mouse actually has a specific go back button on it but I rarely use it.

Thanks for pointing out the Ctrl-Shft-Del hotkey sequence, that's useful. I think everyone should have their browsers set to clear browsing history and all other data when closing. Doing so can prevent SO MANY problems of misbehavior but if not, this key sequence can be used. I try to not suggest corrective steps (with anything) before having some sense of what the problem is. The press and the internet world have so sensationalized random passing security issues that too many think that any PC misbehavior is automatically from a virus or malware. It's usually not the case. Malwarebytes is a fine product. I think it's more efficient to do other things so that its regular use is unnecessary. Better to use tools to prevent issues in the first place than to have an arsenal ready to handle avoidable problems.