It would not be a virus. I'm not having freezing problems yet my computer is quite old (QuickSilver 733 with 1.5 GB RAM & 22 GB spare OSX system drive space.)

Let's take a closer look at your system: System details - help us to help you (click on blue link) & any other information you can provide such as no. of incompletes, etc.

Try deleting your LW preferences folder whilst LW is CLOSED. The prefs folder is found here: /Users/your_username/Library/Preferences/LimeWire
But make a note of settings because once you restart LW, you will need to reconfigure all your settings including your download location, days to keep incompletes, speeds, etc.

If you have a 2nd HDD (or partition), change download location to that instead if that's ok to do so.

If still no improvements after applying the above, then try the following version which is based on LW Pro but is not the official release LimeWire cvs builds (4.13.4jum490) & choose one of the nightly builds (click on nightly build link).