The more recent versions of Windows (including 7) should automatically defrag your hard drive as you work. So defragmenting it now is unlikely to help. However if you still want to give it a go, the route is this:
Start > All Programs > Accessories > System Tools > Disk Defragmenter.
If your computer is slow on everything though, then, almost certainly, something is hogging nearly all of its resources.
Start by running a full virus scan with your anti-virus software.
Then download, install and run the free version of Malwarebytes Antimalware:
https://www.malwarebytes.org/antimalware/
(NB: When installing it, take care to remove the tick alongside the pre-selected option to take a free trial of the Premium version).
Next, go to your list of installed programs. (Start > Control Panel > Programs > Programs and Features). Click on the 'Installed on' column header, to put the list into date order. Look for anything recently installed which could be the cause of the problem. (Uninstall anything you don't recognise. Tell us about anything which you're not sure about).
Then start the Task Manager (via right-clicking on the Taskbar at the foot of your screen). Click on the Processes tab. Click on 'Show processes from all users'. (If you're asked for permission to proceed, grant it). Click at the top right to maximise the window. Click on the 'CPU' column heading, to put the list in order of size. The only high figure should be against 'System Idle Process'. Everything else should be below 10. (Most entries should show 00, 01 or 02). If there's anything unusually large there, that's the cause of your problem. (Ask us if you don't know what to do about it).
Then click on the 'Memory' column header, to put that list in order. Almost everything there should be under 20,000, unless you've got your browser, or other resource hungry software, open. (Firefox is currently showing around 350,000 on my Windows 7 computer, for example). Once again, an unusually high figure is likely to indicate the cause of your problem. If you don't know how to deal with it, let us know what it is.