A few things you can check is:
How much of your total memory used when running the apps you usually have running. If this amount is too close to the maximum capacity you have to focus on limiting what is going on on your computer in a passive state.
How much of the total hard drive is being used. You should at least have a gigabyte or 2 free.
If you have defragmented the harddrive lately(providing its a HDD and not SSD). I can recommend Auslogics Disk Defrag(http://download.cnet.com/Auslogics-Disk-Defrag/3000-2094_4-10567503.html?part=dl-6267754&subj=dl&tag=button)
Did you run the registry clean in CCleaner?
Sometimes your drivers can have an influence on performance, check the manufacturer site for the latest drivers.