Bad monkey! (Greasemonkey, that is. Help me fix weirdness.) (Long.)→[More:]

Okay, so, Saturday morning when I got up and booted the laptop, Firefox was all wonky.

Disabling Greasemonkey made the weird scripty thing in that screenshot go away. However, what followed was two days of really bizarre computer problems. (To be described in a moment.)

I uninstalled and reinstalled Firefox (though it took several tries, and the computer crashed during the installation more than once). I have repeatedly tried to uninstall Greasemonkey. After clicking on "uninstall," it says that the monkey will be gone after Firefox is restarted. Not true. Clicking on the add-on's "disable" or "options" makes the Add-Ons window freeze; have to use ctrl+alt+delete to kill the window.

There's obviously something wrong with Greasemonkey (or Firefox). Main question: How do I fix?

Other problems I have had since this started, which may or may not be related:

1. Navigating to a page with embedded Flash, especially YouTube, resulted in an error message that said "Shockwave has performed an illegal operation. You are strongly advised to restart Firefox." YouTube videos wouldn't play. [Uninstalling and reinstalling the Flash plug-in fixed this.]

2. I was concerned that a virus was causing this, so I endeavored to download anti-virus software. I had a series of problems with these installations:

* Nod32: After installing, I received an error stating "Cannot connect to visual library." Never successfully got the program to run, after repeated installations. Uninstalled it.

* Kaspersky: After several unsuccessful, aborted installations, got the program started. Downloaded the AV database, but afterwards got repeated error messages saying "The databases are corrupted." Updating the database repeatedly did not fix the problem. Could never get the program to run an AV scan. Uninstalled.

* McAfee: McAfee installed with no problem, and I could even get it to scan, but the scan would hang up at file #2200 every time, so it never performed a complete scan. Uninstalled.

*AVG Free: This was the first one I tried, but it took me two days to get it to work. After several aborted installs (sometimes the computer would restart in the middle of installation), opening the program resulted in an error saying "Error launching application." I finally got it installed yesterday, and it [gasp!!] even scanned. No viruses were found. However, the startup components of the program caused my computer to hang up for 10-15 minutes after every boot, so I uninstalled AVG too. (Same for AVG spyware. It scanned, didn't find anything except cookie crumbs.)

[There was also a freak-out hour during all these installs when rebooting the computer resulted in a black screen that said, simply "Missing Operating System." Research showed that this was likely a BIOS problem -- out of the blue? -- or a damaged hard drive. The real culprit, though, was the flash drive that I had neglected to remove. The computer didn't want to boot from the flash drive, I guess. During that hour, I planned the laptop's funeral. Then I finally figured it out. Heh.]

This was a previously healthy, problem-free computer. And suddenly Saturday morning, it's borked. I hadn't downloaded any new software or changed anything anywhere.

My main concern is getting rid of that corrupted Greasemonkey. But if anyone has any ideas on other problems that might be lurking, please help.

Sorry so long.