Windows users should uninstall LW via start > all programs > Limewire > uninstall or C:\Program Files\Limewire\uninstall. If you've tried to uninstall via add/remove programs, that can cause some problems & in that case use your computer's search to look for all files & folders with limewire in the name. Delete what you find.

Are you getting these popups just the once, when you turn the computer on? Or are they popping up randomly all the time? If you're getting constant popups, sounds like you have a worm. This is usually caused by downloading an infected file! There's a fix here to try, it's worked for a lot of people. Click on the link below.
The easy fix for Limewire popping up every few seconds (try this first)

And more info here.
WARNING: Viruses on network you should be aware of!

If you still need help, try one of the sites below...they're more specialised in helping with malware issues

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If you do decide to use LW, always make sure that you're using a good AV program. Keep it updated & use it to scan all files that you download before you open them.