More details on how to do this on various OSes here
- Go to run (Windows + R button)
- type msconfig
- press enter for MSConfig
- In boot click Advanced options.. button
- Check Number of Processors and pick the count
- reboot
After rebooting if you bring up task manager, it will show only one processor instead of usual 4/8:
If you want to revert back to old setting, just uncheck the Number of Processors check box from step 5 and reboot: