Computer uses different kinds of memory to operate efficiently. Memory is required to store the information which may be required for current operations or it may be useful for later usage.
Software Development often includes activities that are not normally associated with engineering, the base of software development. Software Development does not stop at only developing a software product for the client.
If your installation of Windows XP has been there for a while - say 6 months or more - without you doing any housekeeping on the hard drive, then it may begin to get a little sluggish.