afaik , the answer is "mmmm..sort of".
You can load up gba games that have been copied from their carts and converted into files called "roms ", on an emulator (a program that mimics the archictecture of another peice of hardware , in this case the gba.
theres a program called morphgear that emulates the gba as well as the the gb,supernintendo, sega megadrive and mastersystem and a few others.
but!! it runs slowly at the moment (for the gba games anyway), I'm not sure if its playable.
plus the emulation scene is a bit of a grey area as its easy to download roms of games you dont own (which is baaaad mmm 'k :) )
...short answer is , buy yourself a gba if you want gba games right now ..;)
if you want to check out morphgear its available on
www.morphgear.com , its not free ($20) though although you can test games with the unregestered version..