Piriform has a variety of free utilities for Windows that I use and that I recommend to others.
CCleaner - cleans out your temporary files, internet history, cookies, form history, recycle bin, and more. It can also analyze and repair issues with the registry, one of the main reasons why Windows can slow down and have issues. It is a small file and works fast and effectively and I've used it to get many a computer running better.
DeFraggler - defrag your entire hard drive or even just individual files. As Windows files are opened, copied, and moved, the file data can be stored in different parts and on different parts of the disk. This can slow down the reading and accessing of these files. DeFraggler works even better than the built in defrag utility in Windows.
Recuva - Recuva can recover deleted and lost files from your hard drive to save you when you accidentally delete a file, or even some files lost during a computer crash. It can even recover deleted files off of a flash drive, which just saved one of my students from losing some work.
Speccy - Speccy allows you to see all of the system information for your computer. This can be helpful when dealing with problems and issues.
AVG Free Antivirus - free anti-virus and anti-spyware protection. It found some infected files on a computer I was working on that two big name anti-virus systems missed.
All of these are excellent utilities, do their job well, and help you with keeping your system in tip top shape.