To me, it isn't about Windows 7 or Mac OS x, its about "pros / cons" on each for different tasks. And, its also about the cost.
Unknown to some, the iMac system is NOT perfect. It has some great things and it has some "needs for improvement" things. Depending on what your friend does, they may find the "lower cost" Windows xx system better (for their specific usage needs). Not Windows Vista or Windows XP - which has its major problems. Stay with the newer Windows 7 systems - that are much better then prevous Win xx systems.
If wondering, I'd buy a Win 7 Laptop "entertainment grade" system with external 24" monitor the next time around. Thus, having both portability and desktop setup at "affordable" purchase price. And when it blows in 2-3 years, I'd throw it away and get another "low cost" Win xx replacement. Especially for my basic home user tasks. I know. Its like going into a GM forum and telling others I'd buy a Dodge or Ford next time. But truth be told, the iMac doesn't measure up to my expectations (based on market and industry reputation "hype" from others). Especially for my basic home computing needs vrs dollars to purchase it.
Below is something I posted on a different (previous) thread. Based on these items and what your friend does on a computer, they can pick their own system. re: Lower cost Win xx system or much higher cost iMac system?
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Personally, I find my HP Laptop running Windows 7 can access and download internet files (even in wirelss mode) much faster then my iMac connected via UTP ethernet. Even with my iMac's MTU set at its optiminal level, the Laptop/Windows 7 machine blows my new iMac out of the water. The iMac is simply "slow" for movie downloads, You Tube viewing and general surfing - expecially all at the same time.
I find my new iMac for pictures is amazing. Easy to load pictures, easy to view pictures and all pictures are crystal clear. In this area, the iMac is much better then my Windows 7 laptop. With digitial pictures, the iMac actually blows the HP Laptop / Win 7 system out of the water.
For mail, I like our HP with its Windows 7 Mail package better. For automatic spell checker (in mail and within forums), I like our new iMac better. For viewing You-Tube files, the iMac screen is crystal clear. Although, streaming video can be choppy - while waiting for its next download chunk.
For automatic backups, I love the IMac's Time Machine. Set it / leave it and it works. A hear WD Automatic Backup software for Win xx systems is an install it and leave it type usage as well.
For Web caming, I like our HP Laptop / Windows 7 machine "much" better. This bundle includes software that allows you to change WebCam settings. Simple things like Zoom and lighting changes. On the iMac, the iSight Camera is fixed. One needs to buy 3rd party software (re: iGlasses) to change its basic settings. Like gee. Pay $2,000+taxes for an iMac and one has to buy 3rd party software to change their built in WebCam settings? Talk about stupid. Apple Corp needs to shake their head over this gap. Especially since their WebCam pixel ratio (for clearer pictures) is better then other market webcams.
To change fonts on the iMac, one needs to use a 3rd party software as well. re: Tickertool. It changes some fonts but NOT all. And, it doesn't change Menu sizes (like fixed text within a windows label). Within Windows 7 system, simply go into Video Screen Resoluton and select usge large fonts. re: 125% DPI. All fonts and "menus" become larger. Clean / simple and easy to use. To me, the iMac OS needs this same "one global click" feature.
The new iMac don't support Jumbo Frame sizes. With Win xx systems, one can cross reference their onboard ethernet cards and if needed, install different eithernet cards.
Win XX and virus problems?? I've been using Win xx for years and practice "safe computing" (along with FREE Virus software packages - like MS Essentials). re: I don't visit smut sites, I don't click on "free offers" or open attachments from folks I don't know. Never had a virus problem on my Win xx systems.
As you can tell, the iMac is NOT perfect. It has some things that are the way behind the Windows 7 systems. But, are better in some areas. As a suggestion, take your large "user needs" checklist and review the Win xx against the Mac OS system side by side. Which system is better - for you? That's the one you buy. Knowing what I know "today", my family should have bought a "beefed up" entertainment class" Win 7 system - instead of our current iMac. But, we're now stuck with our new iMac - until it breaks or needs to be replaced.
Bottom line... Pick the system that is best for you, and your "user needs". As implied, NO system is perfect. Even the iMac platforms...
Good luck...