I have a T42 running for the last 8 month with Windows 7 ultimate retail edition.
I purchased a newer T61 and I want to know if I can used the retail version of Windows7 ultimate (that I purchased 8 month ago) I was using on the T42 and reinstalling Windows XP on the T42 without any problem with windows genuine requirement. I have the 2 DVD (32 & 64 bits) and of cause the keys and the recovery CDs for the T42.
Thank you in advance for your advices.

You shouldn't have any problem transferring the license of Windows 7 if it's retail (Which I believe from your description, it is.). The worst thing that may happen is you may have to contact Microsoft and activate it over the phone. This isn't a hard process, I have done it many times. You speak to a computer on the other end and it will ask you for numbers and give you an unlock code.

Just make sure to tell it you haven't installed it on any other computers because in essence, you haven't if you have removed it from one PC and wish to install it on another PC.

Good luck mate! Enjoy the T61.

Just to confirm, Brian is indeed correct. Retail license can be transferred, OEM license can not.