With 36G free you are not going to get much mileage out of a weekly defrag unless there is continous unusual activity.
If it is running slow then something is likely chewing the CPU or chewing the memory. What does your task manager say.
Regardless I would
a) uninstall everything you dont need or can easily reinstall.
b) look at your start up processes and stop all the ones not needed. At least switch to manual from automatic and disable some especially those nasty product updaters. Plenty of people if you google offering advice on that including MS themselves
c) Worst of all are the self important virus checkers and such. Unless you good lady is a reviewer of adult site content go with as lightweight a choice as possible. MS Essentials as good as any. In conjunction with a host blocker such as winhelp2002.mvps.org/hosts.htm which doubles as an ad blocker and speeds up browsing perfectly adequate for most people if not for Son of Jobs.