I have been having this issue with my vista machine, getting worse over the last 3 weeks or so. I got to the point of running task manager or process explorer as a matter of course before launching anything else. The task manager did not show any processes taking undue resources when the other windows became "not responding" but only the mouse pointer seemed to carry on working as normal. It did not matter whether I was running just Thunderbird or multiple apps (Firefox, Open Office, WMPlayer, Notepad) the machine is used for email all of the time plus the other stuff on occasion. I never found what the problem was but I noticed, yesterday, that the machine ran from dawn till dusk OK. Today I was out for most of the day but it started and ran just fine this evening. Could be some truth in the suggestion that a driver was to blame, possibly sorted by a new download.
Rans6andrew.