After restart may user requires a password!

Two days ago I’ve formatted my pc and installed a clean WinXp Pro SP3 OS.
After the installation of hardware drivers and an antivirus (Avira) I’ve installed COMODO IS (without AV), all this WITHOUT access to internet.
At the end of installation I’ve launched a complete scan of my system and the program detected some infected files on additional hard drive (not on the clean one with the new XP).
I’ve deleted some of them and I’ve restarted the system.
At the Windows startup the system asked me the password for the user (I’ve got one only user on my computer), but I’ve never used a password for my user!!!
I’m sure that is a CIS issue because I’ve reinstalled Windows, restarted the system several time and my user never requires a password.
So I give CIS a second chance: after the installation I’ve not made a scan of my pc but after system restart now I must insert the user password without know it!!!
What can I do? What’s the password that I must to insert? Can I remove CIS without re-install for the third time Windows?

Here I am with my old pc: can someone help me?
Many thanks!

Somebody else has posted a similar problem, to cure it he used Safe mode F8 to input a password for that account.

Many thanks for your answer.
Can I set a new password in safe mode without know the old one?
Also remove it?
I hope that it works: tomorrow I give it a try…

There is another one Here
Have not found the first one I saw yet.