It seems that you are confusing "memory" with free hard drive space. Once you delete the contents of the temporary folder, you should get the free space back, but System recovery, if active, may keep the deleted files in case you will need to "turn back time". Once you fill up the disk more, the system recovery will discard those files.
WinRAR will also delete it's own temporary files upon the nearest opportunity, so you shouldn't need to do anything manually if you adhere to the rule of closing the setup you were running from the archive prior to the closing of the WinRAR window.
You may also create a new shortcut that would automatically delete the temp folder, like cmd /c deltree /y C:\DOCUME~1\Owner\LOCALS~1\Temp\
but that's risky and generally unneeded.
There are some more utilities that will keep the files not deleted, like All to Recycle Bin or Norton Protected Recycle Bin, but I don't think you have those judging from your question.