Some trojan, virus or other infections. Let it log off, but on the log in screen use the reboot feature and try SAFE MODE. Sorry I never write how to get to XP's SAFE MODE. Please help yourself to that on google.com as this is all volunteer here.
Tell us about the SAFE MODE results.
-> And for more research let's get you started on a bootable antivirus CD. Read and get yours from this link -> http://antivirus.about.com/od/freeantivirussoftware/tp/avrescuecd.htm
The F-Secure bootable CD is free and one of the best.
My Dell Vostro 1510 displays the term "logging off" straight after I press the power button to turn on the laptop. I have no idea why it is not logging on. I thought maybe the battery was low but it still happening even after I charged the laptop up. In between the time I switch on and when the "logging off" term appearing, I have a moment at the bottom of the screen where the it says "press f2 to set up or Press f12 for boot device.." But you have to be quick to respond as it's only stays on the screen for a moment. Does anyone know what to do to get it to log on