So, AVG started flashing me messages about detecting Win/32 viruses all over the place. I scanned, then restarted my computer at AVG's request. I have a feeling that Heur may have infected critical system files, and then AVG deleted them, because now my computer refuses to start up (under safe mode and otherwise). Now, my basic question is, am I screwed or is there any thing I can do to prevent an entire system format? I have irreplaceable files on the computer, and I would prefer all alternatives to formatting.

You should have saved all those irreplaceable files on an external hard drive!! Right now you're basically screwed. Sorry. There have been a lot of viruses going around. I, too, got hit by one and my Norton didn't catch it. I just kept having my computer run slower and slower and slower and boot up slower until it finally just crashed. I have GoBack and the GoBack told me that my computer was unstable and it was not safe to continue. So, guess what? Yep. I had to reformat my hard drive and start from scratch.

You may, I repeat may, take your computer to a computer geek and ask that they try to "back up" your hard drive files for you. BUT, I repeat but, that is not a 100% sure fire guarantee that those files have already been corrupted by the virus attacking your system. I worked at a law office and they did not back up a lot of their files. One day I helplessly watched as the files within my machine were eaten literally before my eyes. It seems that my coworker and I were networked and she had a bad habit of using the email a lot. Plus the boss went on questionable sites. They had nothing backed up. Talk about a disaster! I still have headaches even thinking of how foolish they were in not having an external hard drive system in place backing up everything. Oh sure, they did get a back up external drive - but once they let the horse out of the barn it was too late. Yours just might be the case where the horse ran off and you are stuck holding the empty reins.