Fatal System Error:System has been shut down.

I would suggest running another anti-virus program, however make sure to uninstall or at least disable the antivirus you already have in place before running any of these programs. I would download and install Avira, then I would grab Avira updates to make sure everything is up to date and then I'd conduct a scan. Now if this doesn't work I would download and run Combofix. Just keep one thing in mind when running Combofix, it is a last resort. There may be collateral loss with uninfected files it sees as a threat because it is such a zealous program. It's rare but it does happen. Let me know if this helps.

Yes, it's going to be a simple matter to install another AV, with the machine in a reboot loop. NOT!! AVG may have removed or corrupted one or more infected system files that are critical to the systems stability. If that's the case, boot from your Windows XP disk and do a repair install. Please let us know if you found someone's advice to be helpful.

Well Sci--Guy here's the deal...I've done it before and had success. If he was unsuccessful I was going to suggest he use an online virus scanner such as the one Microsoft offers. My thought is that the OP's antivirus itself may be causing this issue, guess I don't know everything but at least I'm trying to help. As far as his reboot loop goes apparently it stopped or he would have mentioned that it hadn't. @MrHan-Please let us know if you were able to try other antivirus software, and if not does an online virus scan work for you? Let us know.

@Sci-Guy--Thanks for the help. As I remember the error occurred after I updated my AVG and scan was completed and then I restart my laptop. One of my friend also told me, that maybe AVG detected/removed/corrupted an infected files in the system and was disturbed by it. He also suggested that I should boot from my Windows XP disk and do a repair install. I've tried using the Recovery Console, Last Known Good Configuration, or boot and repair using WinXP. But it doesn't work. I still encounter that error. @jmckeeco84-Thank you so much, but how can i run another anti-virus? There is no way of me getting back into windows because that error occurs just as windows loads and so I cant get in. Every time I turn on my laptop (Not the one I'm using right now) I get this message: STOP: c000021a {Fatal System Error} The Windows Logon Process system process terminated unexpectedly with a status of 0xc00000034 (0x00000000 0x00000000). The system has been shut down. This happens every time I attempt to log on to windows, appearing immediately after the Acer and Windows XP loading screen. As I've mentioned, I've tried using the last known good config. recovery console and winXP disk and all attempts to log on was unsuccesful. @jmckeeco84- I've already gave my laptop to my friend so that he can repair it, I will tell him about your suggestion. I will let you know if running another anti-virus or online virus scanner work on my laptop. Anyway jmckeeco84 and Sci-Guy, thank you so much for your help. It was greatly appreciated.

No problem, yeah my next suggestion was going to be to take the hard drive out of your laptop and put it in a shell turning it into an external hard drive so you can scan it. If you wanted you could even grab one at Best Buy, but depending on your PC it might just be cheaper to buy a new internal drive anyway. Hey this is a little off topic but does anyone know how to scan an internal hard drive while it's inside of your computer with another machine? I know it's possible but what is the procedute/software?

Anyone solved the problem already?? I have the same prob. Boot up, and running programs goes perfectly well, however at shutdown returns with BSOD c000021a fatal system error msg. (running XP Pro (32-bit)) Note: I have a feeling the crash code might be caused due HD turned off too early by the OS, while still writing (cached) data... sound of HD spinning suddenly drops...