Computer infected with trojan is constantly rebooting. Is it too late?

About four days ago I plugged an external hard drive into my computer and tried to run a program. It had a trojan, but my computer caught it too little too late and by the time it started announcing warnings, my computer was already sluggish and unresponsive. Immediately after I got the trojan alert, I ran three virus scans: McAffee, AdAware, and Microsoft Web Rooter (or something). My computer wouldn't let me run anything else, and I'm pretty sure it was the trojan as I couldn't open other scans at all via my desktop, nor could I download 3rd party software off of firefox.

I let the three scans run all night and then shut my computer off in the morning, after all three scans coming up with 0 infected objects. I tried using my computer in the night, but it was still wrong: slow, wouldn't let me use certain functions or open certain pages/programs.

Today I went into Safe Mode with Networking, logged in under the administrator user, and downloaded Malwarebytes Anti Malware and ran it. It went for about 36 minutes and found 20 infections, including several trojans. When I clicked to delete them, it said there were a few it couldn't delete and those would have to be added to the "delete upon reboot" list. It then asked me to reboot to finish my deletion. I accepted, and my computer restarted - very quickly - and when it came back up, I tried to go into Safe Mode with Networking. However, suddenly the computer froze and yet again rebooted itself. The fan is blowing loudly, and it has been rebooting constantly for about ten minutes now. I tried starting up normally, but it still only gets midway through the Windows XP home edition loading screen before it goes black and then yet again reboots itself.

I had planned on using Malwarebytes first and then using SUPER Antispyware, and after both I would do a recovery back to last weekend or something. But now my computer is in a constant state of automatic reboot and I don't know what to do.

Is this the trojan in full force? Is it it too late for my computer? If not, can I do anything?

Also - I couldn't do many of the steps mandated in your post about pre-cleaning your computer before asking for assistance here because my computer will no longer log on (or stay on for longer than five minutes.)