Short version of long story:

Elderly Dell PC running XP Pro, taking about 20 minutes to get going in the morning, probably because of Antivirus, Ad-blockers etc.

Too mean to buy a new computer.

Installed Ubuntu 10.04 - quick startup, plenty of excellent open-source software, no need for virus protection.

BUT running several programs on Windows with no Ubuntu equivalent => use of Wine or VirtualBox to manage these, with occasional boot into Windows (e.g. for legacy accounts program).

Already had iPod, two years ago purchased iPad to display jazz charts - not supported on Ubuntu, awkward and slow on VIrtualBox Windows.

The iMac allows me to run all my applications smoothly, with no need for dual-boot to switch between OSs - no regrets!