how many accounts have been set-up on the machine.
Can you remember if you installed or fitted anything before things went awol.
Try pressing F8 when starting and select Safe Mode.
Then if you go into the desktop great, have a quick look round for any unusual icons like AV2009 or Vista2008 these are malware infections.
Try restarting with the bare essential connected. Unplug any printers, Scanners, USB modems.