computer working slow

Most of the time it's been seen and heard that the most common problems that user of computer is 
that their computer works slow
my only answer and reason for their problems is 
"Because of absence of our mind to clean computer on regular basis." 
if you are using windows OS then windows has inbuilt features for most of its problems
Not only this but many other things factor into the lagging of performance, such as where data is fragmented on a disc can effect how your system can find and use it.
 How many different programs are active and running at any given time, if a disc is full and not kept fragmented then data is spread about taking the controller arm on the disk longer to locate all pieces of any given information. 
Virus and spy ware are an extremely common problem as people do not properly know how to avoid them. 
You can avoid viruses by downloading an anti virus software such as AVG. 
Also it depends on the RAM it has. If it has too little RAM then the computer needs to place data it needs to access temporarily on the hard drive making the process slower. If you have too much it will take to long to locate the data and will make the process slower. At least 1 gigabyte to 2 gigabytes of RAM is recommended. 
Furthermore remove all the big files from your desktop. If your hard disk is full get some external USB hard disk and move all your movies and pictures there. Then update you antivirus and anti spyware and run a full scan. After that a DE fragmentation is a good idea. If the problems still persist then you should use a decent registry cleaner.