auto defrag

I looked at the schedule for this last night. every week - Sunday but noted last defrag was 7.7.06 fine as I installed Vista and presumed it defragged after install. Notice showed next date was today but alos showed I didnt need to defrag as everything was in order. I opted to turn off auto as I had read about constant disc thrashing - didn't wwant that.
I was thinking about installing O&O defrag to use insteads as the defrag in XP wasn't the best. I still have to find out if O&O is compatible but have just checked Vista one - tells me I need to defrag. when last night everything was fine - I only installed Vista on Friday & haven't much on it at all - am I best to leave the auot running now or try to install a thrid party such as O&O? I can't believe my system needs defragmented after a few days?