LVitya, open your VboxStartup.log in Wordpad and use the Find tool to look for the word "error". After the first "error" found there are several references to:
"supR3HardenedMonitor_LdrLoadDll: rejecting 'C:\Windows\system32\uxtheme.dll' "
Seems that a good portion of the "error"s found in the log file refer to a bad uxtheme.dll. I'm not too familiar with the security hardening error reports, but my guess is that the theme system on your host has been modified by some sort of theme patcher, perhaps by someone else. You may have to hunt around and find the original 'uxtheme.dll' (the patchers often make a backup). I seem to recall having read that you can bypass the security hardening in Virtualbox by putting the original uxtheme.dll in the Virtualbox installation folder, where Vboxmanage.exe is. (
Might be wrong, but it's worth a try. Apparently not wrong, see
viewtopic.php?f=6&t=70319#p336520) Or restore the original Windows theme system.
There's also a few references to a 'wintab32.dll' which apparently has something to do with a Wacom tablet. Maybe you or someone hooked such a tablet up to your PC once (or you use one now?) and the driver was loaded. It might not be signed? Maybe needs to be removed? Or maybe the theme patch is side-effecting the tablet dll?
Try getting the theme issue dealt with first, then see what happens.