NT4 Workstation

I am quite experienced in installing NT4 Server & workstation, however, for some reason I am having a problem I have never experienced before, I can not sucessfully install a Service Pack. I have tried SP3 through to SP6a. The SP installs ok, although it seems, but up on rebooting SP1 is shown as being installed and then at the logon screen I get an error message relating to spoolss.dll and .exe. I would appreciate any help anyone could give me as this is something I have never come across before.
Thanks in advance.