Occasional pdf-related freeze

T7700 (fast) laptop, 4GB Ram
Vista Business SP1
No weird stuff, very clean, otherwise stable

Three times in the past 36 hours, with no warning, the
unit froze. Cursor disappeared and couldn't be recovered
- tab didn't work, neither did Ctl-Alt-Del. I had to do
a hard shutdown with the power button.

In common to all three events was Adobe Reader opening a
..pdf or reading one, though Thunderbird and Firefox were
both open at the time as well (not "working").

Can you help me diagnose the problem - and perhaps solve it?

Thank you!

Ike