I've had the same problem and it seems to be down to System Resources. Every now and then Xolox grabs a huge chunk of resources and if there aren't enough available it freezes the whole system. I thought it might a leak in the program but it isn't because the resources get released again after about 5 mins or so.

What you need to do is close down anything running in the background that you aren't actually using (like CDr software, seti@home, intellipoint software etc.) Get your resources up to at least 75% before launching Xolox. Oh yeah and don't leave it on the download screen for long periods either, that seems to make the problem worse for some reason. The number of downloads running might be part of this too I'm not sure, but i've managed to run three at once like this all day today.

Hope this helps
Mart