Defragging sorts the disk sectors out in to continous strings of usable diskspace, collecting usable fragments together and making larger areas of usable disk. Reduces disk i/o time.
The second one probably doesnt do much, may offer to delete unused applications or files but you can do that yourself.
Without upgrading all you can do is strip out all tha crap, old applications, achive files, temporary files, cookies, documents, spreadsheets, etc.
Give the PC unita good suck with the hoover through the fan vent, or blow if the hoover has the option, the inside will be like under the bed!