When you start you should have a grub menu onscreen, although it may just
say "press escape for menu" very briefly. On it you can choose "recovery
mode", which is similar to VGA mode in Windows. If that isn't available you
can start from any live CD and edit /boot/grub/menu.lst to enable it. Once
you are started in recovery mode you should be able to change the monitor
driver, in fact you may get a text mode menu that gives you options for
reconfiguring video before the X-Windows starts.
See if you can get further with these suggestions and if you need it don't
hesitate to ask for more detailed help.