I want to use Virtualbox to run Windows in Ubuntu. Can it really run every OS with no problems or restrictions if my PC respects the requirements?
And can I run it in full screen?
No, you can not run every OS. Mainly running "Mac OS X" (for example) is pretty hard, but most mainstream Operating Systems can be virtualised with VirtualBox.
But to answer your questions further:
Operating Systems: Yes, you can run Windows under Ubuntu. I have successfully run Windows XP/7/8 and most of the Windows Server operating systems under Virtualbox on Ubuntu.
Requirements: You will need enough harddisk space for an extra operating system, for windows around 20GB atleast. Also you will need enough spare memory, I would suggest 1GB at minimum. This also depends on the memory requirements of applications you want to run in your guest OS.
Fullscreen: Yes, VirtualBox also supports fullscreen under Ubuntu.
I have been using VirtualBox for testing applications for years its a great tool.
From a SQA perspective I use it too: