You probably don't need to do anything anyway Avast would like you to pay for its optimisation tool. While it's safe to use you probably wouldn't notice much (if any) difference if you ran it.
One of the things it does though (as well as general 'optimisation') is to seek out malware, which is definitely a good idea. To do that for free you can use the free version of Malwarebytes Anti-Malware. (I'd recommend that everyone should have it on their computer and carry out a scan every week or two):
https://www.malwarebytes.org/antimalware/
(When installing it, take care to deselect the option to install a free trial of the 'Premium' version).
If you want to go beyond that, and seek out stuff that might be cluttering up your computer, there are plenty of free programs that claim to do exactly that but I wouldn't trust the vast majority of them. However CCleaner Free is widely recommended by the technical press:
http://www.piriform.com/ccleaner/download
(I've got it on my own computer and I do use it occasionally).
I also like (and sometimes use) the free version of Glary Utilities:
http://www.glarysoft.com/
which is also well-reviewed.
(Don't be put off by the reference to payment on that page. It only applies to the 'Pro' version. The 'Download Now' link gets you the fee one).