You have got a worm (malware) through something you downloaded. Your antivirus will probably not be able to remove it. You should of course try scanning with Antivirus software to see if it is able to remove the worm permanently. If not then try this fix first
Limewire popping up fix - Link.
It may or may not work, depending on the variant of this worm, but is worth a try before seeking more help. In case it doesn't work go to step 5 in this thread (has alot of info on precautions and links to scanners, antiviruses etc. as well)
WARNING: Viruses on network you should be aware of! - Link
or if computer savvy, e.g. feel comfortable editing the registry, these threads may help (just remember that there are many variants of this worm)
How to get limewire to quit popping up every few seconds - Link and
Limewire Acting Wierd (PLEASE HELP!!!) - Link

For more info on how to stay safe:

Antivirusprotection and so on - Link