On the Getting Started Guide ( viewtopic.php?t=1763 ) the first link listed for the Underfoot client downloads ( https://thfmain.s3.amazonaws.com/EQ_Underfoot.zip ) leads to a file that is only 4.3 GB. Once downloaded it wouldn't unzip properly, I had to use 7zip which complained about a bad header but extracted the files none the less, but after updating the client with the XP files and attempting to launch I got error messages about a bad exe. After that I checked on the mirrors and they all have downloads over 6 GB. So I can only conclude that the zip file the first link points to is bad and should be replaced before others run into the same. For now I have the copy from the first mirror downloading instead.
Just for the record, I am attempting to get this running through Wine on Linux, but that shouldn't be an issue. I've been playing on Project 1999 with the titanium client without any troubles for the past few weeks now. I figured I would give the Underfoot client a try on a new Wine prefix so I could go back and forth between the two when I feel like it. I'm guessing it should run fine once I get the new copy done downloading in an hour or two.