There are many possible causes of a slow PC. Because your computer is constantly working with files - moving, copying and deleting from place to place - it will eventually get cluttered with leftover files, some of which are not needed and just take up valuable disk space. You only have a certain amount of disk space available on your hard disk, and as it fills up the computer can begin to struggle to find room to perform its tasks.
If your PC crashes or freezes up, it may be a symptom of a slow or cluttered PC.
The brain of your computer is the Processor, and it may be that your processor is too slow to run today's demanding software. The processor can also struggle if you have multiple programs running at once - for example, if you have Word open to compose a letter while downloading a file from the Internet and also operating another application through your media player.
RAM is the temporary memory the computer uses to do its calculations. If you do not have enough RAM, your programs will run slowly.
Other components of your PC can contribute to poor performance, particularly your Graphics Card when it comes to running media and games software. Some hardware needs special programs called Drivers and these need to be kept up to date.
You may also have programs running in the background using up memory that you are unaware of, and in some cases, the problem may be a malicious program such as a virus or spyware.