I don't think you understood exactly the kind of info I need, so I'll make a few assumptions:

You are probably running Windows XP (Home or Pro, doesn't matter), and you don't have LimeWire set to auto-load when Windows starts. You said there are/were other programs running when you tried ALT+TAB, and switched to each and closed them, so I'm assuming none of them were giving an error message that you had to answer first, and they were all programs you recognized.

When you run LW and this happens, is the Title Bar faded (indicating it does not have the focus)? And how do you usually get out when this happens? Do you just select Reboot from the Start Menu or do you Close LW some other way?

If LW is not set to auto-load at startup, then you need to close all other programs first before starting LW to ensure there is no conflict. If it is loosing the focus, then another program is taking control... very possibly Adware or a virus.

Make sure every non-essential process (except Norton) is unloaded or disabled before you launch LW to see if this changes anything. If not, it is possible Norton Anti-Virus itself is at fault, battling an agressive piece of Adware that it can't eliminate from the system (if you are daring, you can disable Norton too and see if a bunch of Spam pops up after infecting your system. Then use anti Adware software to clean it.)

Try everything (except disabling Norton) first to see if it helps. If not, depending on your Windows comfortability level, you should boot into Safe Mode and have Norton perform a deep scan of all of the files on your PC. It should recognize any adware as a virus, and even if it deletes it, it'll keep coming back.

If NAV does find something, let me know for the next step.