As has been previously reported, some users may have experienced a reboot looping problem when trying to install Service Pack #1 for Vista. According to a post on the TechNet blog, there are now fixes being posted to correct the situation. According to the blog posting:
To address this problem for people who have not already installed the SSU, we are releasing a fix tomorrow which will install prior to the SP1 Servicing Stack Update. This pre-SSU update helps to ensure a smooth install of the SSU by working to prevent the system from rebooting during the SP1 SSU installation. We also made additional changes to the SSU installer code, so that it checks for and requires the pre-SSU (KB949939) before it will install.
These two updates should now install seamlessly through Windows Update, in the proper order, so those of you with WU set to “install updates automatically” who haven’t already installed the SSU don’t have to take any further action. For those using the standalone download of SP1, the issues we encountered do not affect that method of installing at all.
May I suggest that those who have experienced the SP1 reboot-looping problem carefully read the article below. Confirm you understand fully how to resolve this issue before proceeding with any of the recommended fixes. Also note the warnings that Microsoft has issued BEFORE attempting any fixes. It may just save you some time and hassle.
Full article and links to KB fixes are here.