I've also been having this problem. Xolox 1.5 will just "go away", no error messages, or any warning that there might be a problem. I was getting a crash error with the 1.5 betas that would send debug info and allow me to restart, but that has been replaced by THIS annoying problem.

System information:
CPU: AMD Athlon 1.2GHz with 266MHz bus(Thunderbird core)

Memory: 1 gig(1 512-meg modules)

Motherboard: Asus A7N8X Deluxe 1001.C BIOS

Windows XP Professional with Service Pack 1 installed.

I have some problems with IE since I installed SP1, but it won't let me re-install. It's possible that these problems are affecting Xolox. Short of re-installing the OS though, I don't see any way to FORCE IE to re-install. Problems such as windowsupdate not downloading anything, and when in IE not being able to click on anything that opens another browser window are examples of the problems I am seeing. I don't really care since I PREFER Netscape 7.02, but it may be causing problems under Xolox 1.5.