He is old guy and is having problems using vista(I think its 1st version with many bugs).
Umm, what bugs? And, there is no "1st version" unless you mean Vista that hasn't been updated with SP1 (Service Pack 1) yet. Even without SP1, Vista did not have many bugs.
Was he familiar with Windows XP before using Vista? If not, he'll probably find it difficult, also. In other words, he would benefit from some computer training.
Based on what you said about the XP disk - "certificate is one number higher than one stuck on my cpu" - you have, I have the impression you could legally load it on his computer. It could be loaded under a dual-boot configuration, meaning both Vista and XP would be installed on it and either could be used when the computer is started. However, that might just be more confusing for him.
So, you could format C to remove vista and then load xp from the disc. I presume his Toshiba has a recovery partition which would allow reinstalling Windows Vista. I think I'd want to verify that before formatting drive C and installing XP.