Have you found which startup location which took you to that site (e.g. registry "HKLM\Software\Microsoft\Windows\CurrentVersion\Run", registry "HKCU\Software\Microsoft\Windows\CurrentVersion\Run" or your startup folder)? Search that areas for suspicious entries first. Erase (backup first) the suspicious entries and restart (or just log off and log back on). If you still have the problem, re-examine the startup location again.
If the suspected entries is back then there is a running software (malware?) that maintain them. In this case shutdown and start your windows in safe mode, then clear ALL your startup locations (backup first, please).
If the suspected entries do not come back, then you have deleted wrong entries. Just restore back and try to delete another suspicius entries.
You can use my free utility, Start-Q (
Start-Q - Free Windows Startup Manager that Optimizes and Speeds Up Your Login Process) to help you in dealing with startup items (including backup and restore). It will make the above procedure easier