a general rule of thumb is that if you are considering a headunit, to go with a head unit-- the biggest differences is a reliability, boot time, and web.
computers will eventually need maintenance--either because you are like me and tinker with it too much, and finally screw up the os, or because there is a new version of riderunner that needs to be installed
boot time is a very touchy issue for some--i cold boot my pc every time, so it takes about 2-3 min before music is playing-- long enough for me to enjoy the sound of my stock exaust... most people use hibernate for about 20-30 sec. resumes, and now ther is hybrid sleep on win 7 which is supposed to be even faster(10-15 sec? not too familiar with it...)
for some, 2 seconds is too long, for others, 5 min is fine. it is kind of up to you how long you can deal with the voices in your head
also, carputers can have web access-- and, as far as i know, no head units currently on the market come with web access built in (guys using avic units figured out a way to get a web browser, but that is not included from the factory-- so i don't think it counts).