My CanoScan 4200F keeps freezing for no apparent reason - it works for a few
scans and then it gives a communication error then a twain error, I then have
to reboot everything to get it started again, sometimes more than once.  It
did it on my old computer with Windows XP but it now seems to be worse with
Vista Home Premium (on a new Dell Inspiron).  My previous Canon 5000 scanner
gave me a similar problem.  I have downloaded all the latest drivers and
switched cables and power outlets etc. but I still gives sporadic problems.  
When I click on "scan profiles" which is off the scanners and cameras window
through the control panel, it never registers that I even have a scanner
regardless of the fact my scanner icon is in that window and the scanner is
working or not.  This is very confusing to Canon as well.