MY PC Xp & Vista?

Hi, i have Vista on mY Pc and xp i loaded vista today just to see whats it all about and it seems ok.........would there be harm done if i had 2 operating systems??? at the moment i have a option of logging into xp or vista ?? and just a quick feedback do u tink vist ais ok or not worth it as i got it free from my uni?? cheers..