I'm telling the whole story. Apologies if this contains any irrelevant information.
Doing routine maintenance on my father's computer, I removed some spyware with ADWCleaner and went about reinstalling Chrome. Well, when Chrome magically started working again during the install (and behaved like a fresh install), I cancelled the new install. Being short on time, I killed all the Google Updater and other Chrome processes from the Task Manager. I think that's when my problems began.
God knows what Google was doing to XP, but it completely borked all exe files. When I downloaded the Apache 4.11 version of OpenOffice, System File Checker started magically running (with no warning) and consumed 99-100% of my CPU, slowing everything to a miserable crawl. I told my dad to leave the PC alone and let SFC run overnight if needed.
Came back the next day. Trying to install OpenOffice at this time, it gave me this stupid Nullsoft error:
"Installer integrity check has failed. Common causes include an incomplete download or damaged media. Contact the installer's author to obtain a new copy. More information at nsis.sf.net/NSIS_Error"
Well, to fix that, I had to rebuild the areas of the Windows registry that control exe files. That sucked. Yes, I resorted to regedit.
Now my problem is this:
1. System File Checker magically starts up with XP, and sits in the Task Manager. It tells me "Access Denied" when I try to kill the process.
2. I've checked MSConfig and I can find a couple of things in the startup list that I don't know what they are. I leave them checked, lest I uncheck something critical and the system won't boot.
3. System File Checker decides to run, slowing the system down to a crawl. Sometimes, if you want to do something, you either have to work around it, or cold boot the computer. Since I can't kill it, I have to cut the power to turn the beast off during that time.
4. In short, SFC has become the nightmare that will not die. Is there a way to stop it from booting up with XP in the first place?