When I've seen that, it generally has been that something has corrupted windows badly. You may be able to get into windows in safe mode by continually pressing the F8 key as you first power it up. If you know that you just installed something, this may let you get it uninstalled. If it doesn't make it into safe mode either, then it's really hosed. A reload of windows over the top of the existing install sometimes works. That would save your settings, if it was done right.