there are many causes

having too many processes is one of them. Some of the processes could be spyware, which is not the same as "病毒", can slow down your system a lot. You'd better check them out, or show a list and people can check for you.

There are many good processes running but don't do you much good. You can consider closing them. Some of them are from 3rd parties, some of them are XP's services. XP opens many services by default and you can actually close some of them according to your situation and needs.

network printer connection is also a common cause.

you'd also better do defrag once a while.

pagefile can get fragmented pretty bad too. defrag it is a good idea.