The possible causes I can think of are
1) Corrupted DNS cache on the PC (would be solved by a reboot).
2) Corrupted hosts file on the PC.
3) Malicious or corrupted browser toolbar.
4) Other malware, more deeply embedded in Windows.
Those should be detected by antivirus scans.
5) Corrupted DNS cache at the ISP.
If it's not one of those, I've run out of suspects.