If you have to have to have to use MYOB then my vote would have to be Windows 7. I was an XP hangout for years then eventually had to go 7 a couple of years ago with a new desktop and it's brilliant.
Friend of mine died a couple of months ago and had just bought a new desktop with Win 8. I've been helping his Mrs to skill up on the machine so she can do email, internet banking etc and it's a horrible horrible crock of doo doo.
By contrast I recently bought a quad core Android Dongle that hangs off my TV and works off my WiFi, as well as having heaps of grunt in its own right like playing stuff off USB sticks, whatever.
Fairly useless for business applications but for day to day use on the things that most people actually do with their machine like email and surfing RedTube or pirating movies it's stunning. It's my little mate, always remembers what part of a movie I've been on, shows me the pics I've taken on my mobile, goes to the last search I did on my desktop, keeps me logged into AHB etc. Not a bad machine for a free operating system and the unit only cost $90.
edit: I'd second Stu there about alternative open source software. At TAFE and Uni from the mid 90s onwards I bought every version of MS Office from Office 6 to Office XP and became very adept as it was essential to have the spreadsheet, database and Word skills. Since I got Open Office there's not much difference. The best part of buying MS Office was giving me the skills to seamlessly and intuitively use Open Office :lol: