Slow Laptop

I used to use my personal laptop for work,they have since provided me with one just for work.
After i took my personal one home found that it now loads slow and runs really slow. When questioned IT at work was told that Windows was reinstalled straight over everything and that my laptop was not reformatted before this was done.
How do i do a reformat to clean up my computer so that it runs fast again.
Thank you
Di