You only have 4 GB of RAM. CTRL+ALT+DELETE and look at your RAM useage in task manager. If you're at or very close to your 4 GB limit, sort your applications by RAM useage. See what is taking up all your memory useage. Try and close a few non-critical items so that you're using closer to 3.5 or 3GB.
When you say steam is laggy, do you mean your games while you are playing, or navigating the steam client itself? Also are you only using steam when this happens, or do you have a browser open and other programs such as discord or skype?