imac forgotten windows vista password

[4rachel]

My son started the imac up in windows, but we can't remember the password, and we can't switch back to mac. I've tried restarting the computer holding the alt key, but it still starts up in windows. I've tried with the installation disc pressing c but still keeps coming back to windows.
Any idea how to bypass the password or get it back to mac?

 

[Chasingu]

When it starts up, wait for it to do the "DING" noise and then immediately hold down the Option key. It should give you a option on what drive to use.

 

[4rachel]

Thanks but i've tried that and won't work. Do I have to wait til the end of the 'ding', or the beginning?

 

[Chasingu]

Right after. Exactly after. Usually, in the middle will do.

 

[4rachel]

Tried during and after but no joy.