Take Care of The Inside of Your Computer
Before you even think about taking off the cover of your computer case, turn off Windows XP and make sure to unplug the power supply cord from the computer. You can unplug all the other components if you would like. Also, very IMPORTANT, make sure not to work on anything inside the computer if you are on carpet. Move your computer onto a table or any where else but carpet, because carpet produces static electricity when you rub your leg, foot, hand, or whatever on the carpet. Now a good way to ground yourself is to touch your power supply, or other household objects, like a lamp, or water pipe, before you start, or you can get an antistatic wrist strap. This way you avoid ruining your motherboard or anything like that with static electricity, or a static charge. 
Now get you some antistatic wipes to clean the dust or dirt from the inside of your case without touching any surface that contains circuits, or something that looks like your motherboard.
Clean the fan in your power supply, the fan that is connected to your CPU, if you have one, that way you can avoid having your computer overheating. Overheating can make your computer crash, or make it automatically turn off before it completely starts up. So you want to clean this before it happens? Then grab a can of compressed air and blast that dirt or dust out of there. This only temporarily moves the dust or dirt away from these fans. A better method would be to use a vacuum to suck up all the dust and dirt to keep it from clogging your fans up again, or clogging anything else up. Just make sure you don’t let it touch anything, keep it as far from the object as you can as long as you can suck up the dust or dirt, especially with the CPU fan.
If your computer is getting old, over 4 years old, then check those cards plugged into the motherboard to see if they are extremely dirty. 
If so, take the expansion card out and clean it with some compressed air, or a vacuum. But I don’t know how to remove the expansion card! Its not all that hard, just use a screw driver and take out the screw on the left and then pull the card completely out, try not to touch the motherboard if you don’t have a antistatic wrist strap on, or if you haven’t grounded yourself out, which I strongly recommend. Then just repeat this step backwards to put it back in after you have cleaned it.
If your computer is less than 4 years old, then just make sure all your expansion cards are firmly seated. To make sure, just gently press down on the ends on the edge of the cards without touching the face. Also, make sure all the internal cables are firmly connected to where they are suppose to be connected to.
One last thing to do is to make sure your CMOS battery is fine. How do I know it needs to be replaced? 
If your clock on your computer is losing time, this is one of the indicators that your CMOS battery is dieing and you need to replace it. Okay, so now where is it at? It is different on every motherboard, so check your motherboard manual, or search on the internet to find the same exact motherboard you have and see where the CMOS battery is located. But you don’t need to check this battery all the time, just check it if Windows XP is getting towards the 4-5 year mark. Good luck!