I have already mentioned in my post windows xp tools a tool named C-cleaner for this sole purpose. But you can also delete a lot of junk files manually.

These are the places you should try to clear out and it is to be noted that not all the files can be deleted.

• Just open the run prompt (windows key+r) or start-> run prompt and type %temp% to delete the temporary folder contents. Select all the files(ctrl+a) and delete them (Shift+delete).


• Many people mention to delete the prefetch folder contents but i disagree because the prefetch files help launch your applications faster, in fact people change the registry prefetch settings to enable prefetch specifically for application, so do not delete the prefetch files.
  


• Use Windows xp's clean utility :- Open the run prompt and type cleanmgr. Use this to delete the temporary Internet files and you may also want to compress old files because it clears out your hard disk space by essentially compressing old files.


• Open your browser's and delete the history, cache and temporary files, but you may want to keep cache as pages load faster if you visit the same site frequently else you should definitely delete all of them as deleting them makes them run faster and clears out system space.