The cleanup in your browser that Joe suggests should do a pretty good job, but you can't delete a file that's "in use," so the browser can't remove any files that the browser has open when you run it , and the browser cleanup only cleans up junk files your browser has accumulated.
About once per month, the recommendation is that you close all programs except Windows Explorer. (Or do this after a Restart, before opening anything else.) In Windows Explorer, Right click on your system drive (usually C:\), and select "Properties." Click on "Disk Cleanup" on the "General" tab.
Your (Windows) computer will search for files that it can remove and will show you a list of kinds of files it can get rid of. (It may take a minute of two for the computer to make the list.) You can pick what you want cleaned by putting "check marks" in the boxes for each type, or leave things by removing the "checks." Once you've picked which things to remove, the selections should stay the same the next time you do a cleanup, but it doesn't hurt to look at what's checked for removal occasionally.
The difference between doing the cleanup in your browser or using the Disk Cleanup isn't usually too great, but Disk Cleanup is a little more thorough and sometimes helps programs other than the browser that have accumulated some trash while it's cleaning up the browser junk.
Some programs, especially scanner control and image editing programs, accumulate lots of temp files, and Disk Cleanup should get them, while "browser cleanup" won't. The "temp trash" from any program can reduce the space available for all your other programs, including the space for your "browser trash," so cleaning the whole machine is a little more reliable.