There really isn't a downside to reformatting and doing a clean install, but if there was one I suppose it could be that the installation of SP1 might be quite time consuming, especially if you do it through Windows Update. However, there is a way to reduce this time expenditure. If you fall into the category of users that frequently reformat their systems and clean install XP from the CD then creating a slipstreamed version of XP/SP1 can be a huge time saver.....(free).....