help! mac os doesn't work, windows does.

hi,

the other day i installed a printer onto my mac and scanned one document onto my computer using it. when i scanned the second document, my computer froze.

since i know nothing about computers, my friend used the apple care manual for me to see if using the tech tool thing we could find the cause of this problem. the tech tool thing didn't find anything wrong with my computer. then he installed windows, which works now, but my mac os doesn't. it freezes every time after everything loads up (finder and my day) when i turn it on, and the mouse turns into the round spinning thing.

another strange thing is that i have 'my day' (a programme that comes with microsoft entourage), which loads up as soon as my computer boots up. however, in safe mode, i disabled it from loading at start up, but when i start up my computer, it still keeps popping up.

can anyone help me please?!

thanks!!