Over a 5 year life span, cost of ownership is lower than that of a PC.
Let's take an example...
A Dell Dimension - £599 with XP or Vista(before they try to sell you all the crap you don't need)
Over 5 years you'll need to pay Norton £40 every year for virus rubbish
Microshaft Word - £80
As standard no decent photo editor, plus time it takes to defrag. Even though it looks like you get more RAM and storage, think of all the system resource virus software drinks.
Total: £879
iMac 17" 1.83GHz - £679
Includes iLife
No virus softare
iWork - £55
Total: £734
The Mac also comes with a built in camera, bluetooth, Airport Extreme etc... to add all that to a PC adds to the £879 you've already splashed out just to do simple daily tasks.