Follow "best practices":
1. Disconnect from the Internet (unplug the Ethernet cable, or disable the wireless adapter).
2. Backup all your data files to DVD-writable disks, or to an external disk-drive.
Note that the Windows "Files & Settings Transfer Wizard" is useful for exporting your E-mail & contacts to a file -- ensure that your "backup" includes that "export" file
3. Click START
4. Click RUN
5. Type CMD and press ENTER.
6. Type CD "\Program Files" and press ENTER.
7. Type DIR and press ENTER.
to produce a "hint" of the installed programs, e.g., Adobe (Reader), anti-virus, Microsoft Office, and so on.
8. Find all your original CD-ROMs and license-keys, and downloaded programs (such as anti-virus) so that you can reinstall Microsoft Windows and Microsoft Office and the other programs.
9. Reinstall Microsoft Windows.
10. Reinstall Microsoft Office.
11. Reinstall your anti-virus software.
12. Connect to the Internet.
13. Access: http://WindowsUpdate.microsoft.com
14. Click "Upgrade to Microsoft Update".
15. Let Microsoft Update download/install all the latest security-updates for Windows & Office.
16. Reload your data-files from your DVD or external disk-drive.
Good luck! It will take 10 to 12 hours of "baby-sitting" your computer, as it installs & updates Windows & Office.