Do other p2p apps download at reasonable speed? Who is your ISP?
Are all downloads really slow?
Try downloading a file with a few sources, rather than the file with the most sources. Any difference?

Even with Pro, you'll only be able to download from a maximum of 10 hosts (unless the file's a torrent - in that case you can download from more hosts).

Deleting the preferences folder can help with all kinds of problems, so it's always good to try that After you deleted the prefs folder, did you have to go through LW's setup wizard all over again? If you've used really old versions of LW in the past, there could be a folder in the old location of C:\Documents and Settings\your username\.limewire... did you also check for an old folder?

Have you seen the tips here (first few posts by murasame)? A few things to consider while configuring Limewire for optimal performance...

Sometimes a new search can kickstart a slow/stuck file. Don't cancel the original download - just search again for the file. That's helped with quite a few of my bogged files in LW 5.

And if you really hate LW 5, your Pro personal download page has a link to 4.18.8 on the left hand side - 'Get LimeWire Pro Classic'.

Basic 4.18.8 can be downloaded from here: Lime Wire » Download Out-Of-Date Installers