Can't login in Safe mode

I run windows xp in my system and i have installed Symantec Antivirus software. Today it has reported Trojan horse. I have gone through some article for removing Trojan, they have suggested me to boot in to safe mode. However i am not able to boot in to safe mode, it comes up with CNTRL, ALT, DLT screen. I have tried entering the username and password but its not accepting it. I want to know what might be wrong in my case???? I am quite sure that i am entering right username and password.

If its your standalone machine and not domain member one than you can type "Administrator" as username and leave the passowrd field blank for logging in. If its a domain member machine than you might not be having local user account and that's the reason your usual name/password isn't not getting accepted, in that case you will have to contact your systems for logging in as local
Administrator.