There are countless reasons why the system is running slow ranging from hardware to software to configuration problems.
To start I would advise the use of the SFC, (system file checker)
1. Bring up the run command by either pressing the windows key and 'R' or locating 'run' it the start menu
2. Type sfc /scannow
3. Make sure there is a space between the c and the /
4. You will be prompted to enter your Windows CD.
I would then advise a quick cleanup using CCleaner
Use the default cleanup preferences only.
Next a defrag using Auslogics
Make sure you have updated windows using windows update.