For those still using Windows XP and Windows Vista, then here is a somewhat useful service to try out if you run into problems, Microsoft FIX-IT.  Microsoft FIX-IT offer online repair of some of the common problems with XP and Vista installations.

Text from their website;
Thank you for your interest in Microsoft Fix it. We’re working hard to automate solutions to common software problems in an easy, intuitive way that is available when and where you need it. So whether you are looking for a solution in help or support content, or an error report, Fix it provides a way to apply automated fixes, workarounds, or configuration changes so you don’t have to perform a long list of manual steps yourself.