Malicious software is the most common reason for making a computer slow. Continuous attacks over the internet from malicious programs will over time make an unprotected computer very slow. It happens to anyone with a connection to the internet and without any kind of security software installed. It is not a matter of "if", but "when".
Spyware, ad ware, etc are simply small programs and they use up system resources just like your other programs. As time goes by more and more computer viruses will find their way into your computer, if it isn't protected by a security program. Since these programs run as background tasks, they are usually not readily visible to the user. Given enough time this will turn even the most top tuned system into a slow computer. The simplest of tasks will become almost impossible to perform.
Sometimes a brand new computer becomes slow much quicker. This can often be traced to "back doors". These are computer viruses programmed to create a "gate" of such for other programs to enter through. As a result the number of malicious programs accumulate much faster.
Finally your slow computer becomes almost impossible to use and there are only two ways rectify the situation. You can try to remove the malware with an anti virus program. If this fails you will have to completely reinstall your system and of cause make sure to install a security program afterwards.
The slow computer's inability to perform any tasks usually hinders any attempt to clean it computer. Furthermore, with so much malware infecting your system there is a good chance that you have attracted a few that actually targets and disables security software or blocks access to security websites.
So if your computer has become significantly slower, there is a good chance that it is infected with malware. The best thing to do is to address this as quickly as possible. Make sure your security program is fully updated and run a complete system scan - even if it takes a long time. If you do not have a security program you should get one is quickly as possible.