sometimes while in the middle of downloading, I cannot access the Desktop or C drive;

I can still see the Desktop on the screen though; and sometimes all the icons disappeared

To bring them back, I use the 2 finger salute to logoff since I cannot access the Start button

I am not really sure whether MS Security Essentials in the culprit though

Try temporarily uninstalling mse to find out?

like what he said. disable MSE first. then before you download again, scan your PC for viruses. hope you fixed the problem.

Whats he gonna scan with?

Without any specific information about your system, all we can do is guess. Do you have the latest & greatest hardware, or is it a slow single-CPU system, or is it somewhere in between? I've had issues on a fast multi-core PC with Windows Search 4.0 to the point that I removed it entirely.
It all depends on your PC ... You may just have to wait to access the desktop or C: drive until your download is done if your system is underpowered. You may just be using up all your processor power managing the download. To find out for sure, you're going to have to walk away from your PC for awhile and see if the lack of response goes away eventually.

Okay, I'm just wondering why are you trying to access your desktop or the c drive when you're in the middle of downloading something? If you're downloading something while in your browser (like IE8), why not just wait until you're finished if you're having that trouble?

It could very well be possible that your CPU is slow and can't handle accessing different things all at once (if that makes any sense). So far, I've been lucky with that issue. I haven't had a problem with doing multiple tasks such as opening multiple programs at the same time.

You could try opening several programs like Word, Word Pad, Windows Media Player, Movie Maker all at the same time and see if it presents a problem. If it does, then I think it means your processor can't handle it.

This sounds like an explorer.exe crash to me. (opening C: is in explorer, anyways) And anytime I have had the taskbar disappear it is an explorer crash. Did this just start happening? Maybe using a restore point will fix it. And instead of logging off, you can use CTRL-SHIFT-ESC to bring up the Task Manager, select File-Run and type explorer.exe and hit enter. That should bring the taskbar and desktop icons back. But that's only a quick fix, not a real solution.

interesting for info, thaks

I am in agreement with Mike.....

Thanks to give advice.my friend use CTRL-SHIFT-ESC to bring up the Task Manager, select File-Run and type explorer.exe and hit enter. now it is easily bring the taskbar and desktop icons back.