Doesn't even have to be in windowed mode, though swapping takes longer that way.
There are also third party programs (which don't violate the EULA) which support the management of more than one instance of a game running on the PC. Just do a search for boxing s/w for more details.
Edited, Jan 11th 2013 11:18am by tatankaseventh
Tatankaa (90 Inquis)/Urkell (26 Conj)/ Seanthornton (24 Pally), <Inisfree> Freeport server
Tatanka Wolfdancer, 87 Wood Elf Druid
Anaceup Mysleeves, 85 Erudite Mage
Snookims Whinslow, 85 Erudite Enchanter
<Inisfree>, Tunare (Seventh Hammer!)