1. Back Up Your Important Data
The first thing to do? Well, its to back up all your important data in the Thinkpad X61s. I cannot over-stress the importance of this step. If you forget to do this and something happens during reinstallation, well – its gonna be really painful.

Installing Windows XP need not be a hassle
2. Boot Up Using a Windows XP CD
Once all your data is backed up, boot up your machine using a Windows XP CD. Its best to use an original copy of Windows XP rather than some bootleg. And of course, it pays to be legal with these things.
3. Re-install Windows XP
Next, you should navigate to the setup option in the Windows XP CD that allows you to re-install XP. What I typically choose is to select the option to re-format the hard drive and install Windows XP. Once you’re done, click OK and let Windows install itself. Go grab a cup of coffee and wait it out.
4. Install Your Important Programs
Once all your data is backed up, boot Once Windows XP has been installed, you will want to install you important programs. These include stuff like Microsoft Office, Acrobat Reader, email client or other important and indispensable software you regularly use.

Install your important programs like Microsoft Office
Steps 1 to 3 above are more or less generic, meaning you can perform them on any model of laptop. If you are using the Lenovo Thinkpad X61s, then you should also proceed to install your Lenovo Thinkpad software and drivers at this stage (you can usually download these drivers from the Internet).
Conclusion
And that’s it! Once you’ve completed the above steps, you will have successfully reinstalled Windows XP on your Lenovo Thinkpad X61s. Of course, remember that this procedure can be used on all models of laptops – not just the Lenovo Thinkpad X61s. So until next time, good luck and happy computing!