Some crappy files are being deliberately shared by companies who want to sabotage p2p.
Fake files showing up in search results
Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see)

You can use the junk filter to mark those files as junk (right click on the file > mark as > junk), over time LW will 'learn' which files are junk. It's not a perfect filter & sometimes you may get good files being marked as junk. for that reason, it's a good idea to have the filter set to display junk at the bottom search results... that way you can mark good files as not junk if necessary. If you see too many good files being marked as junk, relax the filter a bit (tools > options > filters > junk)

You can also use this hostiles list - it might cut down on the amount of spam that you see.
The TechNutopia Fullsize Hostiles List for BearShare and LimeWire

If all that you see in searches is spam, try changing the ultrapeers that you're connected to.
Click on LW's Connections tab (if you don't see a Connections tab, go view > show/hide > tick Connections). Now right click on a host > remove... a new one will replace it. You can do this for all the hosts that you're connected to.