has this occurred recently?
If recently, any changes on this XP laptop you can think of? EG, Windows updates, anti-virus, or other software updates. Have you had any malware?
Thunderbird on my own Win 7 Desktop is at version 13.0.1 . Do you have the same versions on both machines?
Any add-ons in this Thunderbird? We might think that the same add-ons installed on the two TBs would behave the same, but XP is an older system now and we never know when software is going to become incompatible.
Do you use TB on both systems for the same email account? If so, are you syncing them in any way?