comp locked up windows 2000

when ever i boot up my gateway windows 2000, sect pak 4, it starts to run, and then starts shutting on and off. eventually the screen turns a grey color, and then to blue. the blue window that comes up says"stop(c000021a) fatal system error. the windows logon process system process failed unexpextedly terminated with a status of 0x00000000   the system has been shut down". what do i do. it is my brothers comp and he bought it uses, with no backup disk, or whatever it is called. we know that it is infected, but had still been working. that is till a couple of days ago. now when you turn it on, that is what happens. it keeps repeating the same process, shutting on and off, but never getting booted up and online. help me please?

[Moderator]

Judging by the error messages and the frequency of the issues it sounds to me as if you are going to need to track down an official Win2K CD.....
DLoad the 4 Service Packs on another machine as you will need to re-install them after the repair.

With a Repair Install of Win2K we can save your data and Programs if it's done properly so Post back when you're ready...