Apart from possible malware the main culprit for slow running machines is lack of memory, if you are running XP then I would suggest you have 1GB or more preferably 3GB, anything less than this and your machine will be constantly accessing the hard drive for data rather than caching it within the ram.
Another thing to check is things that are starting up every time you boot the machine. The more that is loaded up at startup the more memory that is eaten up and less is available for the Operating system.