Reformatting a hard disk deletes all data. If you decide to reformat, you can back up all your important documents, data files and photos. The safest practice is not to backup any autorun.ini or .exe files because they may be infected. Some types of malware may disguise itself by adding and hiding its extension to the existing extension of files so be sure you take a close look at the full name. After reformatting, as a precaution, make sure you scan these files with your anti-virus prior to copying them back to your hard drive.
The best proceedure is a low level format. This completely wipes the drive. Then reinstall the OS.
Use the free version of Active@ KillDisk
Or Darik's Boot And Nuke
2 guidelines/rules when backing up
1) Backup all your important data files, pictures, music, work etc... and save it onto an external hard-drive. These files usually include .doc, .txt, .mp3, .jpg etc...
2) Do not backup any executables files or any window files. These include .exe's, .scr, .com, .pif etc... as they may contain traces of malware. Also, .html or .htm files that are webpages should also be avoided.
Download Belarc Advisor
- builds a detailed profile of your installed software and hardware, including Microsoft Hotfixes, and displays the results in your Web browser.
Run it and then print out the results, they may be handy.
An excellent source of Information on this.. Michael Stevens Tech