Downloading a file from Magnetmix is a bit of a test, if you get snail speed there too then it can point to an ISP shaping bandwidth. Sounds like your speeds were pretty good!
Your prefs are probably here:
C:\Documents and Settings\your name\Application Data\LimeWire. It's a hidden folder, so you'll have to show hidden files & folders to see it!

Have you been through the link about configuring LW for best performance? It's worth a try.