When Windows XP just "logs off" it usually has something to do with the registry...often activation, though that doesn't seem to be the case here.
Troj/Krepper-G is Trojan which changes browser settings, downloads and installs/runs new software and modifies the HOSTS file to redirect internet searches.
The Trojan copies itself to the Windows\inetdata folder as services.exe and creates the following registry entries so as to auto-start on user logon or system start.
HKLM\Software\Microsoft\Windows\CurrentVersion\Run\xp_system = c:\windows\inetndata\services.exe

HKCU\Software\Microsoft\Windows\CurrentVersion\Run\xp_system = c:\windows\inetndata\services.exe

HKCU\Software\Microsoft\Windows NT\CurrentVersion\Windows\run = "C:\\WINDOWS\\inetdata\\services.exe"

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects\(5321E378-FFAD-4999-8C62-03CA8155F0B3)

HKCU\Software\Microsoft\Internet Explorer\Main\Enable Browser Extensions = "yes"
that's from: http://www.sophos.com/virusinfo/anal...jkrepperg.html