At this point, you should have a working virtual windows in a window. You may have to do some "windows stuff" like activating (/cracking) windows. Your sound should work, as well as your network
Now lets install the VirtualBox Guest additions by selecting it from the "Devices" menu:
If nothing happens automatically, then in the VM, go to "my computer" and doubleclick on the CD icon. This should launch the wizard to install some drivers that make integration with the host OS much better. Follow the wizard, accept the license, ignore the unsigned driver warnings, etc. At the end, reboot the virtual machine.
Now you can go in and out of the VM with your mouse without it being trapped inside the VM. You can also resize the window, and the VM will automatically adjust the resolution.
You can also map the network drives now, so your VM has access to your real disks. They are available through windows networking. Just create a new network location through the windows wizard and browse to the VirtualBox shared folders (assuming you set them up earlier):
You can also open a command prompt (yes windows has them too! :) and type:
net use x: \\vboxsvr\sharename
where x: is the drive letter you want to assign, and sharename is the name you gave to the location in VirtualBox configuration earlier.
Time for some magic
In the VM, open a window, like IE, or any app, it doesnt matter. Make sure it is neither minimized,nor maximized, then press right-control + L. This activates "seamless mode", which basically means putting the VM in fullscreen and no longer drawing the windows desktop, instead you get to see your Ubuntu desktop. If your windows taskbar gets hidden behind the Gnome panels, try pressing rctrl+L again twice.
There is at least one problem with this seamless mode: it doesn't work with all windows minimized (or no open windows at all). You have to keep at least one windows app open and visible or the screen will go black or corrupt. If this happens, rctrl+L is still your friend. Untill this problem is solved, you can work around it by installing a desklet style app that always shows a clock or something. I run calculator on startup and hide it behind it the taskbar :)
If everything works so far, proceed with step 3 for the finishing touches.