virtualbox.org

The host system: Windows 7 (32-bit)
System in VB: Ubuntu server (64-bit)

It worked good before I tried to install ncurses-hex... (don't remember exactly). The host system rebooted immediately after my first atempt to install this package on ubuntu. After rebooted i started VB and it worked, but again rebooted after second attempt to install this package. At the moment the host system reboots every time I try to start this ubuntu in VB. How to start this VB (it's not essential to install this package but it's important to start VB again)

p.s. log files:

Thanks for the zipped log, it's depressingly unusual for people to provide a zipped log without being asked!

I don't know if this is your problem, but attempting to run a 64bit guest in 800MB seems a tad tight to me (800MB is what you allocated to the VM in settings). Generally people allocate 2GB or more, though I also see that you can't afford that on your host.

It seems very strange... I started VB and choosed memtest, it tested, after that i turned off VB, start again ubuntu and it works! I can't explain how...