Advantages

You may not loose any data or your preferences.

Disadvantages

Only works in about 50-70 % of occasions and any Hard Drive problems are not detected, viruses remain. The success rate can differ from operating system to operating system. Windows 98 2nd edition and Windows ME have higher success rates. Unfortunately you may find that problems get worse or different problems arise from trying this method.

Windows NT, 2000, and XP will delete all files contained within the Windows directory and possibly your user accounts if same username in used

Windows 7 and Vista move all your old data to a folder called "Windows.old"

Updates will need installed again to get system working correctly

With exception to 95,98 and ME all programs will need reinstalled also.

How to do it

Windows ME, 98, 95

You start the windows installation and reinstall in same directory.

Windows NT, 2000, & XP

Just install Windows in same directory as before without a format

Windows Vista & 7

Windows 7 and Vista move all your old data to a folder called "Windows.old", not not format.

 

Extra Notes

Windows 7, Vista, NT, 2000 and XP users have a distinct possibility of loosing all their personal data if they set the login name the same as the previous install.

Windows 7, Vista cannot be parallel installed onto the same partition as any other operating system.