That 0F00:136C error code may appear only because you didn't have a CD in your optical drive(s) during the Express Test, so the optical drive 'fails'. It may not be relevant to your bootup problem.

When you say it fails to boot to Windows, exactly what error messages do you get? What color are the 4 diagnostic LEDs on rear of the tower? If not all green, look up error code in user manual. Installed any new software or hardware lately?

Don't run the Express Tests, run the full tests on your hard drive -this can take some time so be patient. If you get an error message on the hard drive test, copy it down and contact Dell for a replacement hard drive, assuming the system is still under warranty.

If you have to install a new hard drive and load Windows on it, you still may be able to rescue your files by installing the old drive as a secondary drive and copying the files off onto the new drive.

Ron

My mistake, diagnostics LEDs are on front of the 3100. Error codes are here:
http://support.dell.com/support/edocs/systems/dim3100/en/sm/adtshoot.htm#wp1124478

Assuming there are no hard drive errors, and since the kid loaded new software, this is probably not a hardware problem. Have you rebooted and tried "Last known good" from the F8 menu? If that doesn't work... :(

You may or may not be able to boot from external USB drive with XP loaded on it, so that may not be the next thing to try.

Assuming you have XP CD, you could try a Repair/Reinstall of XP. Personal files and software you installed will be ok, but all XP updates will have to be reinstalled. Boot from XP CD and press Enter at the first screen. Don't press R at first screen because you don't want Recovery Console.

After you press Enter, the XP user license should appear. Accept the license and press R at next screen for repair/reinstall. As soon as that's done and you can boot to the desktop, get firewall and antiviral software running. Then go directly to Microsoft update site to install all XP updates and hotfixes again. No surfing and no email until you get the updates installed or you'll get infected with all the malware that's out there.

Since you can't cut the kid's fingers off ( ;) ), once the system is running again, set all other user accounts so they don't have permissions to install/delete software. Password protect the Administrator (and your) account so that only you can add/remove software.

Ron

Reboot and press F2 before XP starts to load to open BIOS setup. Look to see if your hard drive is recognized in BIOS. If it says something like "not installed" or "not found", try clearing NVRAM using the instructions here: http://support.dell.com/support/edocs/systems/dim3100/en/sm/syssetup.htm#wp1053345
Alternatively, unplug the system from the power, press and hold power button front of tower for ~15 sec. Open the case and carefully remove the battery from the motherboard. Press and hold the power button for ~15 sec again; replace the battery (right side up!). Now see if it boots.

If repair/reinstall can't find an OS to repair, then either the HD isn't being seen or something seriously damaged the OS installation.

Good luck!
Ron