You buy cheap software, you get what you pay for. There are already many security holes and bugs for windows 7 even though they already sent out a patch for other holes a month ago. http://www.tomshardware.com/news/Windows-7-Hacked-controlled,7619.html Problems with the install.
http://www.pcworld.com/article/174342/windows_7_upgrade_woes_mount_endless_reboots_and_product_key_problems.html The latest windows update causes the computer to crash. http://news.bbc.co.uk/2/hi/technology/8388253.stm Not only that, Windows 7 is still 30 years behind other platforms, it does not "salt" its password hashes making it easy to get a users password. It's UI may look better then vista, but under the hood it is still unstable. Microsoft makes bad software, Internet Explorer, 7, Vista, 2000, on and on and people still buy it, its amazing.
On a mac, you will not need to install antivirus/anti-spyware/drivers there will be no registry errors, missing dlls and daily crashes. The updates are smooth and upgrades are easy, I just updated from 10.5 to 10.6 with out a problem. Try upgrading from vista to 7 without problems, or even the service packs. I can upgrade from Mac 10.1 (2000) to 10.6 (2009) without data loss and everything working as it should. You can not even go from XP to 7 which is amazing. Not only that, alot of drivers, programs and hardware are not compatible with 7. It is always this way with windows, and you end up spending more money on upgrades and virus removal every time an update comes out.
The applications on mac do not require an installer to install it, its just one icon that can be moved easily. On windows, the installer installs files all over the system for every application, causing fragmentation of the hard drive and the possibility for crashes goes up. This also allows bloat ware and malware to be installed without the user knowing it.
The "mac is unpopular there for not many viruses are written for it" argument is complete BS. With Mac OS 9 back when the mac market share was around 5% there were 500+ viruses for the OS. Today with Mac OS X and a mac market share of 10% you think there would be double the amount of viruses. There are in fact maybe 10 application specific (not OS) trojans for the modern mac. No viruses, just hard to get trojans.