There are a number of reasons why your computer runs slow, one could be that you're infected with spyware or a virus, the other could be that your Windows installation is full of junk and rubbish that is causing your computer to run like a snail in slow motion.
In the hardware world one of the big speed bottlenecks is your hard drive, a hard drive that is not in good health could be the reason why your computer runs slow.
Your hard drive is like any other piece of equipment if you don't look after it then problems will arise, in this case you computer runs slow or maybe even crashes every so often.
Your hard drive needs to be looked after there are a few things you can do to keep it in good working order and your computer at a good speed.
The first one is to defragment it every month or so, fragmentation is when all the sectors or blocks of your drive get scattered all around the disk, this results in a computer running slow and accessing files and programs will take longer, defragmenting ensures that all the sectors or blocks are pushed to one side of the disk so that access to files and programs is much faster.
The second thing that you can to your hard disk when your computer runs slow is to run the Check Disk utility, what this will do is check your disk for bad sectors and then attempt to repair them, check disk can be started by pressing the Windows key and the letter R then typing "CMD" when this opens type in "chkdsk" and then follow the instructions.
In addition to these things it is a good idea if your hard drive is quite old to think about obtaining a newer one, this is good if you have the computer very slow problem, when you purchase a new drive and install it in your computer after reinstalling Windows and transferring your data you should see a great speed increase from your computer.
After you have complete these tasks then your computer should not run slow anymore.
It is a good idea to repeat the first two methods once a month when you notice that your computer runs slow again.