stefan,
sounds like a registry problem. i encountered the same issue last week & solved it finally. in my case i logged in to the pc in question and got the warning message about the paging file, but the pc was on the network, so
from another box, i used regedt32.exe to connect to open the crippled pc's registry.
went to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version
\Winlogon\Userinit:Reg_SZ:C:\WINNT\system32\userinit.exe
i changed that to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version\Winlogon\Userinit:Reg_SZ:userinit.exe
i logged in again to the crippled pc (it let me in this time) and started regedit & went to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices. i right clicked \dosdevices\c and renamed to, i don't know, z, maybe? so it reads \dosdevices\z now. then i renamed \dosdevices\d to \dosdevices\c and restarted the box.
kinda long-winded, but it worked!
-lawrence