i find it harder to track down a specific problem and more time consuming, then to wipe the drive and do a clean install of Windows.
Here's a tutorial
Windows 7 comes with a built-in 'Back Up' feature that allows for a complete System Image Backup or System Image Backup with Data.. and you can even set it up for automatic backups.. it will also allow you to create a Repair Disc incase you can't access Windows.
Before you do a clean install try this
1. Pop in your old HD, and press 'Del', as soon as your computer begins to boot:
- Windows 7: select 'Repair'
- other OS: pop in the Windows CD, let it load, select 'Repair'.