A few other things worth checking:
Check the hard disk activity LED on your PC. If this is steadily lit while the PC is running slow, then there is something making heavy disk usage. For example, if Windows 7 is installed on a hard disk (instead of an SSD), an Antivirus product running a scheduled full-disk scan will significantly affect the performance. This is similar when Windows performs indexing or Windows updates.
If your PC is several years old, try clearing the CPU heatsink with an air duster can. If the CPU is running too hot such as from a heatsink clogged with dust, it will throttle the speed.