I cant get the system to start in any mode.

Hello,
I got an XP Windows security warning. Neither Explorer or Mozilla would allow any internet access and the warning was for a Trojan-BNK.Win32.keylogger. I ran a virus scan and nothing came up but the Windows XP warning continued. I removed Mozilla because that was the first sign of trouble. After that could not get any acccess even with Explorer. Now the system will not start in any mode and upon each reboot the system starts, i see the Windows XP screen and the it just reboots over and over regardless of what mode i use. Ido not have a CD to start from either. Help!