se7en on its own requires about 600MB to load
if you have an anti-virus, you pre-load Firefox or Seamonkey (IE is pre-loaded anyway) & have the updates checks in the background automatically when starting up, well, 2GB may "be gone" in a moment...
also, if the garland of icons bottom right is extending over ½ the screen, well, chances are quite a bit more memory is needed as well.
in fact you can simply see for yourself by starting Taskmgr.exe (press at the same time Ctrl + Shift + Esc) and go to the Performance tab; memory is the lower graph.
go to the Processes tab to see how much memory each application uses (if memory is not displayed, click on View => Select columns; click on the column header to sort it)
you can leave taskmgr run in the background and use your computer as you would normally (when in buntu) and look at the Memory graph in full screen from time to time to see the actual use.
also, Virtualbox is going to advice you what the minimum memory needed for each OS is (look at the se7en config, think it is 1 GB min.)
depending on what you use buntu for, you can adjust that minimum to match your needs.
unless you plan on running a server of some kind, 1 to 2 GB should be fine for most Linux desktop configs leaving you with 2 GB for se7en.
every body should be happy with that ;)
otherwise, memory is quite cheap nowadays and if your computer (laptop?) supports it, add 2 or 4 GB...
then you are definitely out of trouble ;))