what really happened is that you have a fake security program in your computer, which is telling you that your computer is infected with 23 trojans.....and that it's really not infected with 23 trojans.
what this fake security program is--- probably asking you for money to "disinfect" your computer...this is ransomware.
And, the only problem is really only the one (fake security) program, which is really a virus.
Holding down F8 and starting the computer, on a dell, will sometimes give you an option to reset the computer to an earlier time, (restore my computer to an earlier time) before the computer became infected. This is the first thing to try...and try it with 1-4 earlier dates, in an attempt to restore the hard drive back to the time before the virus was installed.
But also...many of the newer viruses disable system restore, so this might not work, but I would give it a try if the option is available under F8.
Another, is to use the XP installation CD to do a repair install, see this forum, fourth post from the top for the directions. A repair install does not erase your personal files, and does not reformat the drive.
I any case, if the computer does start, try to backup all your personal files on to a CD, DVD, thumb drive...so you will not loose your files.
If the computer does not start, take the drive out, install it as second drive on a functional XP computer, drag your personal files off the damaged drive and make a back up with the second computer.
Then, reformat your old drive and reinstall the operating system...reinstall your backed up files.
Of course, if you do not know how to do all this, you can have a computer shop do a data recovery for you.
In the future, don't install free security from the internet, use a real professional all in one antivirus, (one that you pay for).