If your computer is running slow, it could be that you have a virus or other type of Malware compromising your system.
‘Infection’ can occur even if you are careful because the malware often imitates the very software that is supposed to be preventing it!
System Monitor is the best place to start your investigation to see what potentially harmful processes are running and tools such as https://virusscan.jotti.org/ are invaluable to determine if files are infected or not, but sometimes your system is too compromised to do this and removal is often difficult because they imbed themselves into many different files. They may even prevent you from installing antivirus software!
If you think your system may be compromised, do not hesitate to get in touch.