Id start with going to msconfig and disabling any startup programs you dont need. They could be gobbling up your system resources and causing your slowness. Also, if your antivirus has a gaming mode, you can turn that on so it wont interfere with your game while you play.
If none of those work, it could be a possibility your hard drive or another component could be in the beginning phase of failure or you have a virus or malware. Update your antivirus and malware program and run system scans and see if it picks up anything.