It sounds like 2 things, your temporary folder and your temporary internet folder.
C:\Windows\Temp
When your installing software, certain files are needed before the setup can begin, so these files are unpacked into a folder, and the setup continues. However, not all software removes these files when they are finished using them. To resolve this, double-click the windows folder in your C drive. Scroll down the page a little and double-click on the folder called temp. Everything in the folder can be deleted, as it is not needed by windows and is just eating up space on your hard drive. You should periodically check this folder to make sure its empty.
C:\Windows\Temporary Internet Files
When you are browsing the internet, your computer automatically stores the pages you visit into a cache. This is so that pages you visit frequently will load quicker. You need to empty this folder, then alter the settings. To do this you need to open your internet settings... click the Tools tab on your browser, then Internet Options/Settings. A box should appear. In the middle of this box is the settings for your temporary internet files. Firstly press the Delete Files button. This will clear all your cache and it'll free up a significant amount of space. Secondly, click the Settings button. In here you can specify how much space to allocate to your browser's cache. If you have less than a 10Gb hard drive, I'd suggest setting this below 1Gb.