my laptop can't star Window XP home after removed virous

my laptop can't star Window XP home

I go to regedit system to delete all entry that have Ctfmona.exe or LSASS for removing virous-
SASS", "LSASS.EXE" and "Sasser and Ctfmona.exe is Trojan/Backdoor.
However, I can not start window XP home again after shut down.
I have a system CD. But I want to keep all document in C driver and do not like to install
window system again to avoid clean document in computer.
Any way to restart my computer?
使用“故障恢複台”功能進行修複, but ask me to enter password?
My computer is dell laptop and i never set up a password for my computer. do i need to ask dell customer center

Thanks for help!!
Newcomputer