A good virus scanner is a good idea for anyone running a MS Windows machine connected to the Internet (or any other public computer network).

The bad news is that this isn't an installation issue, and you likely downloaded and then ran a piece of malware commanly called the C variant of the "Alcra Worm" (W32.Alcra.C). It's similar enough to another worm that some Anti-Virus programs identify it as the "AN Worm" or the "Zodiak Worm". It appears that the malware was created sometime in mid June of 2005.

The good news is this is that several Anti-Virus companies (such as Kaspersky Labs ) offer free trial versions that should be able to help you out. Also, this issue is covered more in depth in the "General Windows Forums" under Major topics -> LimeWire continually popping up.

At the time of those posts, Norton Anti-Virus couldn't detect W32.Alcra.C. I was able to track down a copy of the worm and send it to Norton. By the end of June, Norton was able to detect this worm. (The June 28th 2005 virus dictionary update was the first Norton AV dictionary to detect W32.Alcra.C, if I remember correctly.)