Sometimes reinstalling Windows can take hours or even days and in some cases it's not possible to reinstall at all in the usual way. But you could make a copy of your entire operating system with disc imaging backup software like DriveImage ,  WinImage , or Acronis True Image  and trials of these softwares are usually available. With a disc image backup the entire operating system can be quickly reinstalled.

Windows CD v. Pre-installed
When a computer comes with Windows "pre-installed" the Windows CD is usually not included. For a reinstallation disc you have to pay extra, even assuming that a reinstallation disc is available. What you actually get with a pre-installed system is an installed "image", a copy of a Windows operating system. In that case there should be a "Restore" CD containing the OS image, but often rigged to only reinstall on the same computer. And some of the extras that come on the Windows CD will be missing. Hopefully the CAB files, at least, are included. These comprise about 100 MB, normally on the Windows CD, and contain backup copies of Windows files. With a pre-installed OS you may find the CABs in Windows\Options, Windows\Cabs or some such.
     If you have the pre-installed version of Windows it's probably also preconfigured with advertising, 100 MB± of outdated ISP software, and various other things to be cleaned out. Considering the time required for cleanup and configuration, and given the fact that you don't actually have a copy of Windows, per se, a disc image backup can be particularly useful.
What is a Disc Image?
A disc image file (or simply a disc image) is an exact binary copy of an entire disc. Disc image files contain complete data stored on the source, including not only its files and folders, but also system-specific data. Actually, a disc image is not a collection of files or folders, but is an exact duplicate of the raw data of the original disc, sector by sector.
     Since disc images contain the raw disc data, it is possible to create an image of a disc written in an unknown format or even under an unknown operating system, such as those used on DVD players.
     File formats of disc images usually depend on the type of the source drive. For example, ISO files are CD images. Disc images are widely used by CD recording software for transferring and storing complete CD contents. A lot of CD recording programs use their own formats for disc images. Some of these formats are plain ISO image files renamed with another extension.
To back up Windows with minimal risk of data loss
  • Install windows.
  • Install all software, drivers, updates, etc.
  • Configure everything the way you want it.
  • For pre-installed windows: copy the CAB files to at least one external medium. The CABs can then be deleted if you need to save space.
  • In Win98 - If you use System File Checker it would be good to run it. By so doing the record of changes can be cleared, making SFC easier and more useful later.
  • Do a Defrag.
When everything is set up the way you want it ...
  • Make a disc image backup.
  • Store it on CD(s) and/or another partition.
  • Make 2 or more copies of boot floppies for the disc imaging programme.
Maintenance
If you have multiple partitions/hard drives, multiple copies of important files can be saved in different places. Copies of patches, updates, etc. should also be saved as they're collected.
On a regular basis ...
  • Make backups of important files.  Keep all work files in single folders and copy whole folders to backup. For example, a folder for graphic work, a folder for word processing docs, etc. It is also a good idea to make REG backups, using RegEdit.
  • Go to Windows\Application Data\Microsoft. If you use Outlook Express you should find all of your mail and your address book in subfolders there, along with files relating to various other programmes.
    Now copy the whole Microsoft folder to a safe backup. 
Reinstalling
When reinstallation is required ...
  • Boot the disc image programme.
  • Install the backup image and boot it.
  • Replace Windows\Application Data\Microsoft with its backup.
  • Replace other backed up files and folders.
  • If hardware changes have been made it will be necessary to update the software for that.
  • Any changed or added passwords will also need to be re-entered.
That's it ... in less than an hour you should be back where you started with a fresh system.
     This whole process is easiest if there are multiple partitions set up. Then everything except programmes and Windows itself can be kept on other partitions. When it comes time to reinstall, only the main OS needs to be replaced. Some sources recommend a separate partition for programmes. That will be problematic if disc image backup is used.
     Any programmes installed since the image was made will be unregistered in the fresh version of Windows. Their registry entries will be missing and in most cases it will be necessary to reinstall them or to use REG backups you have made.